Things get even more complicated when the sending has to be done from an app you or your team created, requiring the integration of the email-sending functionality.

To tackle the challenges of mass email sending and the integration of the email-sending functionality, using an email API (Application Programming Interface) is the best route. In this article, we cover what an email API is, its benefits, usage, and more. So strap in and enjoy the read!

What is an email API?

An email API is a type of web API typically hosted on a cloud email delivery service that aims to connect an app, platform, etc., to an email service provider (ESP)/email platform. By doing so, it enables easily accessing data and features of the ESP/email platform. 

Some of the features email APIs provide access to include:

• Email generation and sending
• Email template generation and editing
• Bulk email sending
• Scheduled email sending
• Personalization 
• Email list management
• Email metrics analysis
• Deliverability and spam management
• Webhook creation for sending real-time email event data
• Incoming email parsing 

On top of accessing the mentioned features, with an email API, you are also enabled to operate at scale, as email clients (Gmail, Outlook, Apple Mail, etc.) will no longer deem your behavior to be spammy in situations when your sending volume spikes. This is due to the fact that API calls go through cloud servers, and email APIs, in general, can have layers of authentication. Pretty impressive, right?

How does an email API work? 

For many, just a simple description doesn’t cut it when trying to understand how something as complex as an email API works.

So let’s break down a common use case, shall we? 

The whole process begins when a user interacts with, for example, a web application. Let’s imagine this interaction is as simple as placing an online order.

After that, the application sends an email API communication containing the order details as well as the user’s email address. 

This communication is, technically speaking, an API request sent through available endpoints. And prior to accepting the request, the email API will do an authentication process in which it checks for a correct API token.

If the correct API token is found, the email API then gets to work crafting an email message using a template, the order details, and anything else communicated to it.

Once complete, the email is sent to the user’s email address through an SMTP connection with the recipient’s MX (Mail Exchange) server. And in order to be able to generate email analytics, the system that provides the email API tracks and records the user’s interaction with the email, including the delivery, opening, clicks, and so on.

Finally, after processing the request, the email API will send a response using HTTP status codes, typically in JSON format, although they can also come in XML, YAML, or other formats.

Types of email APIs 

In broad terms, there are three categories of email APIs that serve different purposes – transactional email APIs, contextual email APIs, and email marketing APIs.

Transactional email APIs

Transactional email APIs are used to design and send automated emails which get triggered by events or user actions. The emails sent by these APIs are transactional messages such as order confirmations, receipts, password reset emails, and other email notifications. And the most important characteristic of transactional email APIs is their reliable and timely delivery of time-sensitive and/or important messages.

Contextual email APIs

Contextual email APIs, on the contrary, are used for email marketing messages and notifications with high levels of personalization and dynamic email content based on user behavior or contextual information. Using them, engaging and targeted email marketing campaigns can be created as content is generated dynamically at the time of email rendering.

Email marketing APIs

Lastly, email marketing APIs are intended to allow devs to integrate email marketing features into their apps, projects, etc. Through them, interaction with email marketing solutions is enabled. This way, the features of said solutions can be accessed through code rather than a GUI. Some of these features include:

• HTML template email sending
• Contact management
• Email list management and segmentation
• Email marketing campaign management
• Contact campaign list retrieval
• Campaign list retrieval
• And more

Is using an email API better than using SMTP?

Answering this question requires us to first state one important thing – email API and SMTP (Simple Mail Transfer Protocol) represent two different methods of sending and receiving emails.

How different are they exactly? 

Well, for one, SMTP is a protocol, or in other words, a set of rules and guidelines instructing how communication between two email servers should go when a message is being delivered. 

Anyone who uses email clients or applications, like Outlook, is simultaneously also using SMTP as the clients or applications rely on the protocol to send emails to an SMTP server and then to the recipient’s email server.

An email API, as mentioned at the beginning of the article, is a set of tools and methods intended to enable developers to integrate their apps or other projects with an email-sending functionality. 

When using an email API, developers get to enjoy a high level of abstraction, meaning they do not have to deal with any low-level details of email-sending protocols. 

So, which method should you go for? Well, that depends on your requirements and skills.

For those who don’t want many customizations or additional features, SMTP will suffice while also providing them with a very simple setup process.

But if lots of features and more control over the email-sending process are what you are aiming for, then an email API is what you need. Just keep in mind that, depending on the API you go for, you might have to make some significant code changes to integrate it.

What is an email API provider?

Email API providers are nothing more than companies or services offering an email API and any infrastructure, tools, and resources needed to integrate the API.

The providers handle everything from message assembly and sending to reporting, so you or your dev team don’t have to do it alone. They also work towards minimizing interruptions affecting email delivery which can happen due to outages, malicious attacks, network problems, and natural disasters.

So, if you choose to use an email API provider, what can you count on? A streamlined API integration workflow as well as a reliable service! On top of that, as email API service providers send millions of emails monthly, and actively keep spammers off their platforms, they (in most cases) also come with good reputations and email deliverability.

How to choose an email API provider

Not every email API provider can fit all teams out there, and choosing the wrong one can make a world of difference when it comes to landing in your recipients’ inboxes.  

To help you avoid a headache, we recently researched the top email API providers, focusing on flexibility. That is, the provider’s ability to send your emails reliably while allowing you and your team to fine-tune your sending process. 

So, we sent emails with each API, tried different sending configurations, set up webhooks, played around with templates, and more, all the while taking the following into consideration:

• Infrastructure and scalability 
• Sending logic and performance
• Integration and extensibility
• Templates and personalization
• Customer support

As a sneak peek, here’s a brief flexibility comparison between Mailtrap and Resend:

To learn more about our methodology and get more in-depth information on technical tidbits like IP infrastructure, throttling, retry logic, etc., read our full comparison. 👀

What are the benefits of using an email API?

Although, by now, you most probably have the idea that email APIs can be very useful for devs in charge of adding the email-sending functionality to apps, platforms, etc., some of you reading this still might be a bit hesitant to use them. And that is totally understandable, especially if this is a new concept for you.

So to help you better understand the benefits of using email APIs, we’ll now go through the most notable ones:

• Simple integration – Integrating an email API can take just minutes for those with advanced knowledge of coding, as the process with most email API providers is simplified and streamlined. What’s more, API documentation and SDKs necessary to get you started are also almost always provided.

Learn what an SDK is and how it simplifies integration:

• Faster email delivery – When using an email API, there is less required communication between servers, resulting in a faster delivery time. On top of that, email APIs, when using HTTP, are accepted by most firewalls, thus removing an additional obstacle.
• Great security – Email APIs come with the requirement of needing an API key to use their features. This way, when integrated into your app or platform, they protect it from phishers or spammers. 
• Access to analytics – Keeping track of your email performance metrics, such as the number of delivered and rejected messages, open rate, click rate, etc., is key. And unlike SMTP, email APIs provide insight into these metrics. 
• Ease of use – After setting up an email API and integrating it, you have essentially made your email-sending process automated. And if you need to send tons of emails at once, the API should make the process quick and easy. Also, depending on the email API you decide to go with, you might also have access to a GUI for your not-so-tech-savvy team members who want to manage contact lists, edit templates, manage emails, and so on.
• Better deliverability – Email APIs from good providers are built to reduce the chances of a message failure and ensure (almost) every message reaches the recipient’s inbox, which is not the case with manual email-sending methods. This is largely thanks to email API features, such as delivery optimization, error handling, and reputation management, as well as their robust infrastructure, documentation, and support.
• More efficiency – Email automation is why most people choose to use an email API. This automation can reduce resource usage as well as save time and boost efficiency overall.
• Scalability – Handling large volumes of emails is what email APIs are designed for. So, in cases when your email traffic increases, your email API should be able to scale to accommodate this and ensure reliable delivery.
• Integration with other services – Besides your app, email APIs can integrate with different services, such as CRMs, marketing automation solutions, e-commerce platforms, and more. 

How to use an email API?

To demonstrate the usage of an email API, we will use the reliable and hassle-free email API provided by the Mailtrap Delivery Platform, an email-sending solution for developer and product teams, focused on fast delivery and high inboxing rates for transactional and promo emails.

With Mailtrap’s email API, you can deliver emails to recipients just in time, as it gives you a stable email infrastructure with high deliverability rates by design. What’s more, thanks to its unique monitoring features, you can effortlessly control how your email infrastructure works and troubleshoot unexpected sending issues using this email API.

The mentioned features include:

• Helicopter-view dashboards
• Drill-down email reports for mailbox providers 
• Extended email history with historical data up to 60 days 
• Email body preview after the email was sent 
• Weekly reports with week-on-week comparison 
• Critical alerts if numbers suddenly drop

The users of this RESTful API also receive other benefits, such as ≈ 1 sec email delivery time, a secure and effortless setup and verification process, as well as the option to switch to using an SMTP service depending on your project’s needs.

So, what are the steps for setting up and using Mailtrap’s email API?

First, you create a Mailtrap account.

Then you add and verify your domain by adding five records to your domain’s DNS, as described here.

With a domain added and verified, you then need to integrate the API with your application. 

To do this, find the API and SMTP tab, choose the API option, and select a code snippet from the list of integrations in different programming languages. The detailed API docs can be found here.

It’s important to mention that Mailtrap does offer the following SDKs to integrate their email API into your application:

• Node.JS SDK
• Ruby SDK
• PHP SDK
• Python SDK

Add the code snippet to your app or project and run it. Also, make sure to click on “Verify Setup”, so Mailtrap can check if your test email has arrived.

If yes, you should see the following screen:

You can then go to “Email Logs” and see the details of the email you just sent with the email API.

With Mailtrap, you also have the option to enable tracking settings, unsubscribe settings, and webhooks. For more information, check out the dedicated knowledgebase page.

Note: If you’re more of a visual learner, be sure to watch this video on getting started with Mailtrap’s email API.

After completing all the above steps, Mailtrap’s email API should be all set up.

What is the difference between an email API and an email server?

As we’ve defined what an email API is a few times throughout the text, we won’t repeat ourselves again. Instead, for this comparison, we’ll simply define email servers and then give a breakdown of the differences in table format.

So, an email server represents a software application (hosted on shared servers or cloud platforms) or a physical server which is the infrastructure handling email sending and receiving, storage, as well as routing. Think of it as a central communication hub just strictly for emails.

Without this type of server, email communication would be impossible as it’s what processes email sending by checking the recipient addresses, routing the emails, and finally delivering them. And when it comes to any incoming emails, an email server stores them until they are retrieved by recipients.

What do email servers use to make all of this happen, you may ask? Well, for sending emails, good-old SMTP is used. POP or IMAP is the protocol used for retrieving emails, and MIME is used for handling any attachments or multimedia in emails.

And how does one interact with an email server? Well, you don’t do it directly. Instead, you use an email API, which allows you to integrate the features of an ESP into your app or project. And behind the scenes, this ESP relies on an email server to do the actual email sending, receiving, and storage.

To help solidify the difference between email APIs and email servers, take your time to study this table:

Say goodbye to the complexities of email protocols by choosing the best email API

Learning about email APIs is the easy part. Picking which API to go for is where things get tough. 

What the right API for sending emails will be for you will depend on a number of factors, such as your needs, technical knowledge, audience size, suitable pricing, and so on.

But if we can give you any advice for when you are evaluating the options, that would be to look at high-performing and robust APIs which can also be customized. 

Of course, most, if not all, of the features we listed earlier in the article should also be present in your email API of choice, as only that way will you be able to add complex email-sending functionality to your app or project without much knowledge of all kinds of email and telecommunications protocols. 

For more content like this “What is email API” tutorial, make sure to explore our blog, where we cover everything from email infrastructure and security to HTML emails and email marketing.

We also have a YouTube channel on which we tackle topics such as “APIs vs SDKs”, so give that a visit while you’re at it:

ESP migration: a checklist

Before we dive into technical and not only details, here’s a quick checklist you can use to plan your ESP migration:

Audit your current ESP setup:

• Review sending setup, email volumes, audience structure, and email types.
• Map out all integrations, webhooks, and API callbacks.

Prepare your data for migration

• Export suspension and contact lists, templates, logs, automation workflows, and custom fields.
• Document DNS records (SPF, DKIM, DMARC) and subscription forms.

Set up your technical infrastructure at the new ESP

• Add and verify your sending domains.
• Configure SPF, DKIM, and DMARC records.
• Warm up your IP or domain gradually if needed.

Configure access controls and security

• Enable two-factor authentication (2FA) for all users.
• Assign role-based permissions to control access to key areas.
• Enable and review activity logs for auditability.

Set up integrations and webhooks

• Gather new API tokens and update connection settings in your systems.
• Reconnect CRMs, automation platforms, and other tools.
• Set up and test webhooks to handle engagement events in real time.

Test in staging, then check deliverability and inbox placement

• Run test sends in a staging environment to validate sending logic and system behavior.
• Use tools like Mailtrap Email Sandbox to inspect messages and debug flows.
• Send live tests to real inboxes to check rendering, tracking, and placement.

Plan the timing and phasing of the migration

• Choose a migration window with lower sending activity.
• Migrate in phases — by email type or audience segment.
• Communicate the plan across dev, marketing, and support teams.

Monitor post-migration performance

• Track bounce rates, inbox placement, and key engagement metrics.
• Set up dashboards and alerts to catch anomalies early.
• Watch for deliverability issues and unsubscribes over time.

Now, let’s see how to change your email service provider step-by-step.

Step 1. Audit the current ESP setup

Yes, ESP migration is a big deal, yet with a clear plan, you can change the provider without breaking operation, data losses, and deliverability drops. 

By this point, I assume you’ve already done your homework and know exactly which ESP you want to migrate to. If not, it’s worth checking our guide on how to choose the email sending provider for your specific business needs, or check these articles:

• Best SMTP Provider: 7 Platforms Compared
• Find Out Which Mass Email Service Provider Is Right for You 
• Free SMTP Server: 10 Providers Compared 
• 7 Best SMTP Services for Email Marketing

Already know your hero? It’s time to analyze how your current setup works. The goal is to document what’s in place today, so you can plan the migration with fewer risks and no missed steps.

Look at:

• Audience management — how your lists and segments are organized.
• Sending volumes — number of emails sent monthly, sending scale and limits.
• Types of emails — marketing, transactional, triggered marketing, or bulk campaigns.
• Current deliverability and placement — deliverability rates, inbox placement stats.
• Integrations — CRMs, ecommerce platforms, CDPs, automation tools, custom APIs, anything connected to your ESP or triggering emails.
• Webhooks and API callbacks — check what events (bounces, opens, unsubscribes) are pushed to your system and how they’re processed downstream. 

The gathered information will help you understand what will move, what might need reconfiguring, and what you’ll need to test after the migration.

Step 2. Export current data

After the audit, export the data you’ll need for the migration to make sure no valuable information gets lost during the switch.

Here’s what you need to export:

• Suspension list. Suspension lists include email addresses of unsubscribed recipients, those who bounced, or marked you as spam. You need to export this list and import it into your new ESP so you don’t accidentally email the wrong people. Otherwise, your deliverability rates and sender reputation will hurt.
• Contact lists. Download your email lists to quickly re-import them into the new ESP.
• Logs. Export campaign logs, deliverability reports, engagement stats, and bounce rates. These will be important for measuring performance after the migration.
• Templates. Export your templates in both raw HTML and your current ESP’s proprietary format (if applicable). Some platforms use custom variables or markup that will need to be adapted to the new ESP’s syntax.
• Email performance data. Export historical campaign performance data (open rates, click-through rates, conversions) for reference and consistency in reporting.

Additionally, you can also save:

• Automation workflows. Export or document triggered email workflows, automation sequences, and drip campaigns so you can reimplement them in the new ESP.
• Custom fields and tags. Save any custom fields or tags used for segmentation and personalization.
• Sending domain authentication. Document SPF, DKIM, and DMARC records to recreate proper email authentication in the new ESP.
• Subscription forms and landing pages. Export any subscription forms or landing pages that are integrated with your current ESP for re-implementation.

Step 3. Set up your technical infrastructure and warm up your domain

Now that you have your data ready, it’s time to configure the technical aspects of your new ESP. 

If you’re migrating at a large scale, you can request migration support from your new email service provider to streamline the process and be sure all your specific requests and needs are taken into account. Mailtrap, for example, offers dedicated support for large-scale senders, helping you with the transition and smooth infrastructure setup.

With or without external help, focus on these key tasks:

• Configure sending domains. Start by adding and verifying your sending domains in the new ESP. Use the SPF, DKIM, and DMARC records you documented earlier to configure these in the new platform. This step will help keep your email deliverability rates safe during the migration.
• Validate your SPF record. After configuring your sending domains and updating your SPF record, use tools like the Mailtrap SPF Record Checker or Kitterman SPF Validator to make sure your domain is correctly authorized to send emails from the new ESP. 
• Monitor DNS propagation. DNS changes can take anywhere from a few hours to 48 hours to fully propagate. During this time, some email services may still see your old records. Use tools like MXToolbox or DNSstuff to monitor the status of your DNS updates.
• Warm up your domain/IP. Many modern ESPs provide automatically warmed-up IPs, but if you’re managing your own dedicated IP, gradually warm it up by sending small batches of emails to highly engaged recipients. Slowly increase the volume over time to avoid spam filters and keep your reputation high.
• Review and adjust bounce handling settings. Test how the new ESP classifies and handles bounces. Check whether soft bounces are retried and how long suppression entries persist. These behaviors vary across platforms and can impact deliverability if left misaligned. Also, check how bounce data is exposed through webhooks or logs, especially if your system automatically updates contact statuses based on those events.
• Set up your tracking for deliverability and engagement. As you configure your domain and warm up your IP, set up the dashboards and reports in your new ESP to track deliverability, engagement, and sender reputation. This will give you a clear view of how things are going from day one.

Step 4. Configure 2FA and user access controls

Before you start sending emails, make sure your account is secure. Enable two-factor authentication for all users. This adds an extra layer of protection by requiring both a password and a secondary form of verification to log in.

Next, review permission levels within your new ESP. Many platforms support fine-grained roles that let you control access to key areas, like sending settings, contact lists, analytics, or billing. Assign roles based on team responsibilities to streamline workflows and reduce the risk of errors or data exposure.

Finally, set up activity logs to track user actions within the platform. These logs help you monitor who’s making changes, accessing sensitive information, or updating configurations. The feature helps you detect issues early or prove compliance with data regulations like GDPR checklist, APA, CCPA, and others.

Step 5. Upload your data and setup integrations

Take the contact lists, templates, and other important data you exported earlier and upload them into your new ESP. In some cases, your email templates may not import directly, and you might need to rebuild them using the new ESP’s tools.

Connect your tools and platforms, such as CRMs, marketing automation systems, or eCommerce platforms, to your new ESP. This helps keep your workflows running smoothly once you begin sending live emails. ​​For smooth operations, you need to set up:

• SMTP Integration. Double-check that your SMTP credentials are correctly configured in the new ESP. Verify the host, port, username, and password to match the new ESP’s requirements for proper email delivery.
• API Integration. If you use an API for sending emails, update your API tokens and keys in the new ESP. Confirm that the API endpoints are correctly set up to prevent disruptions in your email flow.

If your new ESP uses webhooks for engagement tracking, set up and validate those endpoints early on. Webhooks are often used to push real-time events like bounces, unsubscribes, opens, or clicks to your system. Make sure your receiving endpoint is authenticated, can parse payloads correctly, and handles retries or malformed data. Run test events (if the ESP allows it) or simulate webhook calls locally to confirm that data is being received and processed as expected. This will help avoid missed events or broken reporting once you go live.

Step 6. Test the setting, email deliverability, and inbox placement

Start by simulating your real sending environment, run test calls through your API, trigger automated workflows, and validate integration points with your CRMs, databases, or backend logic. Focus on confirming that the right templates are pulled, variables are populated, and emails are successfully handed off to the ESP. If you use SMTP, double-check authentication and error handling. If you rely on an API, verify response codes, retry logic, and payload structures.

You can run these tests in a controlled environment using tools like Mailtrap Email Sandbox. It allows your dev team to confirm that sending logic works as expected, without worrying about inbox delivery or user-facing content just yet.

Once your system behaves as expected, move to testing email deliverability and inbox placement. Start with a small batch of live sends to different mailbox providers. Check how your emails render in real inboxes, verify that tracking links work, images load properly, and unsubscribe links are functional.

You can also use Mailtrap Email Sandbox for it. ​​Sandbox functions as a fake SMTP server and allows you to inspect full email content and headers without sending anything to real recipients. 

Then, use inbox placement tools to analyze where your emails land, inbox, spam, or promotions. This gives you early visibility into potential filtering issues and helps fine-tune your domain or IP warm-up process.

Step 7. Schedule the final transition

This step will help you check there’s minimal disruption to your ongoing operations. Here’s what to begin with:

• Define the migration window. Choose a time frame for the migration that works best for your team. Depending on the scale of your business, it’s best to pick a window when your email volume is lower or when you’re not running any critical campaigns. 
• Break it into phases. Don’t migrate everything at once. If possible, break the migration into smaller phases. Start with a small segment of your audience or migrate by email types (e.g., transactional first, then marketing), test everything thoroughly, and gradually expand. This way, you can address any issues without impacting your entire user base.
• Communicate with your team. Make sure everyone involved in the migration is on the same page. Developers, marketers, and customer support teams should be aware of the plan and ready to react quickly if something goes wrong. Clear communication helps minimize downtime.
• Prepare for troubleshooting. While you’ll have done plenty of testing, be prepared to troubleshoot any issues that arise during the migration. Have a plan in place to monitor deliverability, inbox placement, and any technical errors. The faster you can identify and resolve an issue, the smoother your transition will be.
• Have a fallback plan. If something breaks during the cutover: an API fails, delivery drops, or emails start bouncing, you need a way to recover fast. This could mean pausing campaign sends, temporarily routing high-priority transactional traffic through your old ESP, or switching DNS records back until the issue is resolved. Fallback options help protect your customers from outages and give your team space to fix problems without panic.

Step 8. Monitor post-migration performance

Once your migration is complete and you’re live with your new ESP, you need to regularly monitor key metrics to make sure everything is functioning as expected. Keep a close eye on:

• Performance. Track open rates, click-through rates, and conversions. These are the signals that your emails are performing well in terms of engagement.
• Deliverability. Monitor inbox placement and bounce rates to check for any deliverability issues. Keep in mind that your new ESP might have different settings or infrastructure, so deliverability could vary compared to your previous provider. This is a normal part of the migration, and tracking these metrics will help you spot and address any changes.
• Errors. Monitor all types of email errors, including those in transactional, triggered, and marketing emails. Check for issues in reporting, API connections, or integrations that might impact email delivery or performance.

Set up automated alerts or dashboards to monitor spikes in errors, bounce rates, or delivery delays. The first few days after migration can show some changes, but you need to monitor for at least a couple of weeks to get a true sense of how things are settling.

Watch out for bounce rates and unsubscribes after migration, too. If these numbers start to go up, it could signal deliverability issues with your new ESP. Spotting and fixing them early helps support your critical business communications.

Wrapping up 

If you’re not seeing the growth you need because your current provider can’t handle your increasing email volume or you’re facing constant issues with inbox placement, it’s time for a change. With the right planning and tools, you can switch providers smoothly, without major workflow interruptions or declining email delivery rates.

Considering Mailtrap as your next ESP? These detailed step-by-step migration guides for popular providers will be very helpful:

• A Step-By-Step Guide on Migration from SendGrid to Mailtrap 
• A Step-By-Step Guide on Migration from Mailgun to Mailtrap 
• Mailchimp Transactional Email To Mailtrap: A Migration Guide 
• A Step-By-Step Migration Guide from Amazon SES to Mailtrap 

Thankfully, some early email specialist figured this out a long time ago and came up with the concept of IP pools – the smart way to keep different types of email traffic in their own lanes so things don’t break as sending grows.

In this article, I’ll walk you through what email IP pools are, why they matter, and when to implement them. I’ll also dissect what might be causing poor deliverability in your IP pool, as well as what you can do to get things back on track.

What are email IP pools?

An IP pool is a group of IP addresses assigned to specific email streams – categories of emails that serve a similar purpose, like marketing campaigns, transactional messages, or user alerts – with the goal of creating stream isolation.

This isolation helps ensure that one stream’s issues, like low engagement or high bounce rates, don’t drag down the deliverability of another stream, as each one comes with different sending patterns and recipient behaviors.

Depending on the email sending provider (ESP), email pools can be created for a range of different purposes, including:

• Separating transactional and marketing emails

Transactional and marketing emails behave in distinct ways and carry unique deliverability risks. So, using separate IP pools (and ideally separate domains/subdomains) for each stream ensures mission-critical emails, like password resets or billing receipts, land in inboxes consistently, even if a sender’s latest promo takes a hit.

• Managing high volumes 

Sending a large volume of emails from a single IP can lead to delays and throttling. With IP pools, you can spread the load across multiple addresses to avoid volume spikes, reduce bottlenecks, and keep the email flow smooth, even during heavy send periods.

• Isolating client traffic

If you’re managing email for multiple clients (such as in a marketing agency or as a reseller offering email services), you don’t want one client’s issues dragging down everyone else’s. IP pools let you assign a dedicated sending infrastructure per client, so you can isolate performance, monitor reputation, and troubleshoot without cross-contamination. This kind of setup essentially mirrors multi-tenancy in email sending, where each client gets their own sending space, protected from the deliverability risks of others.

• Grouping by sending reputation

When onboarding new senders, their reputation might be unclear. With IP pools, providers can isolate senders into streams such as “New”, “Trusted”, and “Needs Attention”, based on what they know (or don’t). This helps shield the cleanest IPs from risk, while still giving new senders a place to build trust.

Disclaimer: Before you get too deep into how IPs are grouped and managed, keep in mind that this breakdown reflects how email service providers typically structure IP pools behind the scenes. As a sender, your reasons for needing IP pools might be different, and we’ll cover those next. Still, it’s helpful to understand how things work in the backend so you can make smarter decisions when choosing an ESP or planning your sending setup.

For instance, Mailtrap users can take advantage of separate IP pools to create transactional and bulk email streams. This ensures that high-urgency messages, like login confirmations or billing notices, reach inboxes quickly and without interference, while newsletters and promotional campaigns can be optimized and managed independently. Both reach recipients, just with delivery strategies tailored to each message’s purpose.

Why use email IP pools

Okay, so I’ve covered the reasons ESPs implement separate IP pools. 

Now, it’s time to tell you why you, as a sender, might want to start using them. And no, this won’t be a comparison between shared vs dedicated IPs, as there already is a separate article on that topic on the Mailtrap blog.

Instead, I’ll focus on the practical side of IP pools – how they help isolate risk, optimize delivery, and gain control as your email sending grows.

Reason #1 – To improve email deliverability 

When you use the same IP to send all of your emails, the underperformance of one campaign is very likely to hurt all of your other messages as well. 

So, say you sent a marketing campaign and it got low engagement. If you now use the same stream to send your password reset or billing alert emails, chances are the low engagement of the marketing campaign will stop the inboxing of your high-importance communication.

That’s because mailbox providers evaluate sender reputation at the IP level, not just per message. And with Gmail, Yahoo, and Outlook rolling out stricter protections against spam as well as new requirements for high-volume senders, keeping streams isolated is more important than ever. 

Beyond isolation, IP pools promote consistency, something mailbox providers reward. You see, by using IP pools, you can assign high-engagement messages their own IPs while having completely separate IPs for newer or more volatile campaigns. This way, your best-performing traffic builds reputation quickly, while riskier sends are contained and monitored without threatening overall deliverability.

Reason #2 – To manage sender reputation

As mentioned a few paragraphs ago, sender reputation lives and dies at the IP level. And when you’re sending different types of emails to different audiences, lumping everything together under one IP complicates how ISPs determine this reputation.

With IP pools, you can give each stream its own space to build a reputation that reflects its actual performance. And if things go wrong, and maybe engagement dips or complaints happen, each stream is isolated in its own IP pool, and the damage doesn’t spill over. Think of it as risk management for your sender reputation, and IP pools as your safety net.

IP pools also give you more hands-on control over how sender reputation is maintained, as they allow you to apply stream-specific practices, like setting tighter bounce thresholds for re-engagement campaigns, or enabling closer monitoring for riskier sends like last-minute promotional blasts.

Reason #3 – To support high-volume and scalable sending

ISP throttling, delays, and rejections can all be results of sending high volumes through a single IP. With IP pools, email load can be spread across addresses, reducing bottlenecks, even during peak periods. This distribution of traffic also works great for staying under ISP volume thresholds and not exceeding any daily or hourly per IP caps. 

Now, in terms of scaling, IP pools prove especially useful as they allow you to add and warm up IPs in a controlled manner within pools. So, if one IP fails or gets temporarily blocked, other IPs in the pool can take over, thus maintaining performance and adding resiliency.

Lastly, for use cases involving fine-tuned delivery behavior, such as rate limiting, custom retry rules, or timing controls, IP pools serve as an ideal mechanism to enforce stream-specific configurations and thus optimize for urgency, user experience, or mailbox provider preference.

When to use email IP pools

So, you know what IP pools are and why you need them. But what are the situations when you actually should leverage them? 

Here are the key scenarios where IP pools become your strongest line of defense for deliverability:

You’re sending high volumes

If you consistently send large volumes of email, especially into the hundreds of thousands per day, using a single IP can lead to delays, rate limits, or poor inbox placement. Also, as ISPs tend to flag unusually high-volume traffic from a single source as suspicious, switching to IP pools to distribute volume more evenly is the way to go if you want to stay under the ISP radar.

Looking ahead, if you’re a new business that has plans to send massive, diverse campaigns, having IP pools set up should be the first step to take if you want to enable concurrent sending across multiple IPs, increase your throughput, and improve load balancing.

When it comes to bulk senders whose IP has already shown signs of throttling, soft bounces, or erratic performance, this behavior should be a clear indicator that it’s time to switch to using IP pools, as any of the mentioned things often mean your current IP is straining under the load.

You’re sending multiple types of emails

As most businesses usually send a mix of transactional, marketing, as well as other message types, and can observe their varying complaint risks and engagement patterns, using IP pools is recommended in order to give each its own stream.

IP pools also provide you with more flexibility to use different strategies or SLAs for each stream. So, if, for example, you need 30-second delivery for login codes or maybe want to pace out a re-engagement campaign and not your other messages, this level of control can be applied thanks to the IP pool architecture.

You’re introducing a new email stream

If they have been established for a while, your existing transactional or marketing streams likely have solid reputations. A new stream, however, can carry unknown risks, such as different content, untested audiences, or experimental strategies. 

By assigning this new stream a dedicated IP pool, you can ensure that any hiccups won’t affect your existing infrastructure. This means you’ll essentially be creating a sandbox environment for testing the new stream setup, all while protecting your core IP reputation and not jeopardizing inboxing.

And if the new stream requires a gradual warm-up, the same approach will work as the multi-IP pool infrastructure gives you space to complete the process properly, with low, steady volume that builds trust.

How to use email IP pools

Management of IP addresses significantly varies from ESP to ESP. But what most reputable ones have in common is that they don’t give you direct control. Instead, they let you create streams or subuser accounts to segment traffic, which are then internally mapped to specific IPs or pools.

Mailtrap applies the stream-based approach in order to give users a strategic way of leveraging the platform’s IP pool infrastructure. 

And here’s how you can create separate email streams within Mailtrap:

1. Verify your domain 

Under the Sending Domains tab, click on Add Domain and enter your details in the input field.

Next, you should be taken to the domain verification page, where you’ll find the DNS records that need to be given to your domain provider. 

For more details on this step, check out the sending domain setup guide.

1. Integrate the Mailtrap stream you want to use  

Depending on your use case, transactional or marketing, select the corresponding stream option from the Integration tab.

Then, for sending via SMTP, copy the credentials, such as the host, port, username, and password, and paste them into your project, app, email-sending service, or any platform that supports the protocol. 

Alternatively, you can pick a programming language or framework from the menu under “Code Samples” and copy the sample configuration already containing your credentials. 

For sending via API, toggle the switch to API, build the authenticated HTTP request in your programming language or framework of choice, and configure it with the Mailtrap host and API token.

Just like the SMTP route, this one will also give you the option to choose a programming language or framework from the menu under “Code Samples” and copy a configuration with your credentials integrated. 

Under the same tab, another thing you should see is the official SDKs for PHP, Python, Ruby, and Node.js, which can simplify integration and accelerate setup. So, if you’re working within a supported tech stack, be sure to take advantage.

Once you’ve completed these two steps, your new email stream should be ready to send at scale with high deliverability.

Also, thanks to detailed analytics being just a click away, with Mailtrap, you’ll always know how your email streams are doing and where there’s room to improve.

When your IP pool might be causing poor deliverability

While it’s true that deliverability issues often stem from factors like poor email list hygiene and spammy content, sometimes the culprit is structural.

So, let’s talk about some common ways your IP pool setup might be working against you.

Different types of emails are sent from the same pool

As I covered earlier, many providers won’t ever clearly disclose how they handle stream-level IP isolation. This means that even if you’ve set up distinct “streams” or “categories” in the UI, those may still be bundled together under the same IP pool behind the scenes, all in the name of resource optimization.

The problem? Well, say your IP pool sends both high-engagement password reset emails and bulk marketing blasts that flop. To a mailbox provider like Gmail, it all simply looks like inconsistent engagement from a single source. This, of course, can lead to consequences like throttling, filtering, or even spam placement for your most critical emails. But worst of all, it can completely defeat the purpose of using (and paying for) IP pools in the first place.

The IP pool wasn’t properly warmed up

With automation at their disposal, and IP assignment being a tedious task, many providers prioritize speed, automatically assigning IPs to accelerate setup. However, without proper warm-up routines, your emails may be sent from cold IPs that haven’t yet earned trust with mailbox providers, hurting deliverability from the very start.

To top it off, ESPs might also skip engagement-based warming, failing to prioritize the most active subscribers first.

As a result, your IPs may struggle to maintain a strong reputation, and even with solid content and a clean recipient list, you could face throttling, soft bounces, or frustrating delivery delays – all problems that could’ve been avoided with a better warm-up process.

The IP pool was oversegmented 

Although it’s tempting to go granular – one pool for every stream, client, or campaign – overdoing segmentation can also create its own issues.

You see, when providers underutilize IPs by sending only a few thousand messages per month, those IPs can’t generate enough volume to build a strong reputation. And if your sending is spread across too many IPs, it not only drives up costs but also stretches resources thin, making it harder to monitor performance and detect deliverability issues.

Your role in managing IP pool performance

Sure, your provider manages the IP infrastructure, but that doesn’t mean you can be completely hands-off. As a sender, your actions still play a critical role in maintaining strong deliverability and overall performance.

To get the most out of your IP pools and safeguard your reputation, here are some proactive steps you can take:

• Understand your provider’s setup. Ask if transactional and marketing streams are routed through separate IPs. If not, see if there’s an option to separate them.
  
• Monitor your metrics closely. Look out for sudden drops in open rates, spikes in soft bounces, or previously reliable emails landing in spam. These may signal IP-level issues.
  
• Check your sending practices. Make sure your lists are clean, engagement is steady, and new campaigns follow warming best practices. If you’re doing everything right and still seeing problems, your IP pool might be the issue.
  
• Talk to your ESP. If you suspect you’re in a poor-performing pool or that your transactional emails are suffering due to other traffic, contact support. A good provider will investigate and may reassign your traffic, adjust your stream configuration, or warm up a new IP.

Wrapping up

And that concludes this deep dive (pun intended) into email IP pools.

As a final tip, I want to remind you that even if your dashboard shows separate streams, it’s your provider’s responsibility to ensure those streams don’t end up sharing the same IP behind the scenes.

At Mailtrap, we take that task seriously, implementing isolation at the email infrastructure level to make sure your delivery logic stays intact and your emails land where they should – in the inbox.

If you’re ready to put it into practice, click below to get started with Mailtrap.

If you want to do more reading on email deliverability, here are some other articles you should check out:

• 10 Email Deliverability Tools and Marketing Platforms 
• 7 Best Tools for Testing Email Deliverability 
• Where to Find an Email Deliverability Consultant and What to Ask 
• Email Domain Reputation: All You Need to Know
• Email Sending Reputation Audit 

In this guide, I won’t just list features, I’ll walk you through how to think about choosing an ESP based on what your business actually needs. I’ll cover everything from deliverability and pricing to sending infrastructure, design tools, integrations, and analytics, explained in plain English.

Best ESPs: a snapshot

Before we dive into the tech details, here’s a quick cheat sheet to help you narrow down your options and help you focus on some specific platforms, not all of them at once:

• Free or low-cost ESPs: MailerLite, Moosend
• For high-volume senders: Mailtrap, Postmark, Amazon SES
• For marketing automation: MailerLite, Kit, HubSpot
• For high email deliverability: Mailtrap, Postmark
• For irregular senders (occasional campaigns, seasonal use): Mailchimp, MailerLite

While some of these tools brand themselves as email marketing platforms, email delivery services, or customer engagement tools, I prefer a more practical classification — if a tool can send marketing, transactional, and bulk emails, I consider it an email service provider. 

Now that you’ve picked out 3, 4, or maybe 6 options, it’s much easier to go through the technical and strategic criteria without getting overwhelmed.

On that note…

How to choose an ESP: start with your real needs

Let’s be real, price is usually the first thing people check when picking an email platform. And fair enough. But here’s the catch: you’re not just paying for a plan, you’re basically paying per email that actually lands in someone’s inbox. Some tools look cheap on paper… until you start dealing with poor email deliverability or extra features, you don’t actually need.

Most ESPs come with a bunch of plans and fancy features, but there’s no one-size-fits-all. What works for a growing SaaS company could be way off for a small e-commerce store or a startup that sends a handful of emails a month.

So before diving into comparisons, take a minute to map out your current email needs:

• Are you mostly sending transactional emails like order confirmations or password resets?
• Planning to send marketing emails campaigns, newsletters, promos?
• Do you send regularly, or just once in a while?
• Is your list small and engaged, or are you sending at scale?
• Do you need advanced features like automation workflows, predictive sending, or security compliance?
• What other tools are you using and will you need to connect them all together?
• What’s your team’s level of experience with email platforms?

Now zoom out and think long term. If you’re planning to grow your list, send more emails, or run bigger campaigns, make sure your email platform can keep up, without blowing up your budget.

Also check if the platform includes extras you might need down the road, things like automation, segmentation, or product integrations. You might not need them on day one, but having them ready to go means you won’t be forced to switch tools when things start to scale.

Choose email infrastructure that won’t let you down

When you compare email sending tools, it’s easy to get caught up in features and pricing. However, beneath the surface lies the email infrastructure — the technical component responsible for delivering your messages to subscribers. 

If the infrastructure is weak — say, it doesn’t have proper backups, uses low-reputation IPs, or lacks failover systems — a chunk of your emails might get rejected by receiving servers or get straight into spam folders. On the flip side, a solid infrastructure gives you high deliverability, better sending speeds, and peace of mind during peak times. Here’s how to select a good one.

Pick a platform with high deliverability rates 

You could write a perfect email, but if it ends up in spam, you’re looking at lower engagement, wasted budget, and lost opportunities. So, as you compare platforms, here are the things you’ll want to look out for:

Want help evaluating all these needs for your company? Mailtrap provides free email deliverability consultation for business and enterprise-grade customers.

Check if a platform lets you scale 

Some tools are great when you’re just getting started but hit their limits fast. Scalability is about whether your ESP can keep up as your list grows, campaigns get more complex, or you start sending way more emails. Switching ESPs down the line can be painful. Picking one that scales now can save you time and headaches later.

Go with a tool that won’t flake under pressure (reliability)

Some emails are mission-critical. If your password reset or order confirmation doesn’t go out right away, it’s not just annoying, it could cost you customers. Downtime or delays can hurt your business and your brand’s credibility.

Check the sending capabilities

Some ESPs focus more on transactional emails, while others are built for marketing campaigns, or try to do both. Here’s what to look for if you need to send transactional, marketing, or bulk emails.

Can it send transactional emails?

Transactional emails are time-sensitive. It’s not enough to just send them, they need to arrive fast. 

Can it send marketing emails?

Marketing emails are traditionally the money makers for any business, they convert leads, move them down the pipeline, re-engage inactive users, and accelerate sales. Modern ESPs come packed with tools to save you time and help you send targeted, personalized campaigns.

Can it send bulk emails?

Bulk emails are great for announcements, promotions, and newsletters. The challenge? They can overload your system. Look for an ESP that will make high-volume sending smooth.

Mailtrap is an email delivery platform designed for product companies with high sending volumes. Go for high deliverability, growth-focused features, and industry best analytics.

Pick a platform that makes email design easy

Even if you’re only sending transactional emails, they still need to look sharp and work on any device. The right ESP should make email design fast, clean, and flexible, without a learning curve.

Find out if an ESP supports AI tools and integrations

ESPs, like many tools today, use AI to automate tasks, optimize campaigns, and improve email personalization. Integrations are important too. They connect your email tool to the rest of your stack without duct tape and dev time.

Check the analytics and reporting tools

If you can’t see what’s working and what’s not, you’re flying blind. A good ESP should give you clear, useful data, not vanity metrics or cluttered dashboards. You want reporting that helps you track performance, catch problems early, and make better decisions fast.

Make sure the platform covers your legal bases

Nobody gets excited about compliance, until something breaks. Your ESP should handle the legal stuff behind the scenes: adding unsubscribe links, tracking consent, keeping user data secure. You shouldn’t have to cobble together fixes or worry about fines later.

Check the customer experience

It’s not just what the tool does, it’s how it feels to use. A clunky dashboard, half-baked docs, or zero setup help will burn hours fast. Your ESP should feel smooth from day one, not like a puzzle your team has to solve.

Check the overall user experience

A solid platform doesn’t disappear when things go wrong. Great customer experience shows up in how a company handles problems, responds to feedback, and keeps you in the loop when stuff breaks.

Make sure the pricing fits your usage

Everyone checks pricing first, but choosing a platform just because it’s cheap can cost you more in the long run. I put it last for a reason: pricing only makes sense once you know what you’re actually getting. The real price shows up when features fall short, deliverability tanks, or support vanishes.

Email service provider selection cheat sheet

Here’s a quick recap of everything we’ve covered in this guide. This cheat sheet breaks down the key criteria to look for when choosing an ESP, so you can review, compare, and decide faster. Download it, save it, keep it handy.

Wrapping up

Choosing the right email service provider isn’t about finding the most popular name, it’s about matching the tool to your needs. I hope this guide helped you navigate the decision with more clarity.

To make things easier, I’ve put together some articles that dive deeper into specific ESPs, giving you clear comparisons and insights:

• Find out the best SMTP provider: 7 platforms compared
• Find out which mass email service provider is right for you
• Find out the best free SMTP server: 10 providers compared
• Find out what’s the best email API service for developers
• 6 best email marketing platforms: tested & compared

They’ll give you a closer look at specific categories and help you narrow things down even further.