Let’s face it: there is no email API that fits the needs of every team out there. However, a solid API will give you control of your sending process and allow you to fine-tune it according to your team’s requirements.

In other words, an email API needs to be flexible.

So, I’ll break down the email API flexibility for 5 of the best providers out there.

First, I’ll give you a snapshot of my research, tell you about the methodology I used, and then I’ll cover the following components (click to jump):

Disclaimer: I’ve spent hours researching every email API flexibility component, so you can expect a genuinely unbiased comparison.

Email API flexibility comparison: a snapshot

Click on a platform name to jump ahead to the detailed review.

Mailtrap has the most flexible setup, offers separate sending streams, a full-fledged Sandbox, advanced webhooks, making it great for high-volume senders.
Resend offers great flexibility but lacks some features like dedicated streams, testing solution, and templates, which can be a deal breaker for some.
Postmark is a reliable, minimalistic choice, but if you plan to use it, you’ll have to handle retries, templates, and testing on your own.
Mailgun offers a plethora of advanced features and is quite flexible, although it lacks dedicated streams, and its customer support availability depends on your plan.
SendGrid is a super flexible option for users who are willing to pay for a higher plan for more control of their sending infrastructure.

To give you a sneak peek, here’s a table (one of many) that summarizes the key findings of my research:

Provider	Dedicated IPs	Separate streams	Rate handling	SDKs	Webhooks	Templates	Testing
Mailtrap	Available from 100k emails/month with auto warm-up.	✅ Bulk and transactional	✅ No limits, customizable throttling	Node.js, PHP, Ruby, Python, Elixir, Java	✅ Full events + logs	✅ Handlebars	✅ Full Sandbox + API
Resend	Available as an add-on from 500 emails/day with auto warm-up.	❌	✅ No details on throttling	Node.js, PHP, Laravel, Python, Ruby, Go, Java, Rust, .NET	✅ Limited, domain-focused	❌ Manual	❌ Test addresses only
Postmark	Available as an add-on from 300k emails/month	✅Transactional and broadcast	✅ No limits, auto throttling	Ruby, RoR, .NET, Java, PHP, Node.js	✅ Standard events	✅ Mustachio	✅ Included in plan
Mailgun	Available from 50k emails/month as an add-on and included from 100k emails/month.	❌	✅ Yes, limit not specified	Python, Go, Node.js, PHP, Java, Ruby	✅ Full events	✅ Handlebars + versioning	✅ Two separate products
SendGrid	Available from 100k emails/month.	❌	✅ 10,000 r/s, 1,000 recipients/email	C#, Go, Java, Node.js, PHP, Python, Ruby	✅ Full events + logs	✅ Handlebars + versioning	✅ Sandbox + features vary by plan

\Features are relevant at the time of writing.*

Methodology

First, I analyzed the official documentation. This includes support portals, knowledge bases, changelogs, and various integration guides from providers, like this one for example.

Going through these docs, I was relatively able to see how easy to use an API is, what it allows me to do, and whether it’s maintained regularly.

Then, of course, I ran tests with each API to simulate real-world usage. By doing this, I was able to see how much control I had over email sending, how easy it is to set up an API, etc. The tests I ran include:

Sending emails with different sending configurations to test real use cases
Triggering rate limits and retry behavior to see how the API handles high volume
Create email templates and see how much personalization they allow
Setting up webhooks for some of the events they support
Using the testing solutions to validate my email designs, check spam scores, etc.

The most important information regarding APIs and the tests I ran is in the following chapter.

Email API flexibility detailed comparison

Infrastructure and scalability

Let’s start off with the essentials, that is, the API infrastructure, for which I considered the following:

Scalability
IP infrastructure
Email streams
Multitenancy support

Scalability

An email API is scalable if it allows you to increase your sending volume without delays, deliverability hiccups, or infrastructure bottlenecks. To allow you all of this, most modern email API providers offer cloud-based infrastructure paired with other features like dedicated IPs.

The following table sums up the technical tidbits related to scalability:

Email service provider	Scalability
Mailtrap	Cloud-based infrastructure, multiple MTAs, deliverability experts for custom setup
Resend	Cloud-based infrastructure, dynamic scaling with dedicated IPs
Postmark	Cloud-based infrastructure, multiple load balancers in different regions
Mailgun	Cloud-based infrastructure, Rapid Fire Delivery SLA
SendGrid	Cloud-based infrastructure, distributed load balancers across the globe, scaling with multiple dedicated IPs

What it means:

Mailtrap offers automatic MTA distribution and help of deliverability experts for setting up your sending configuration. This makes it best for teams who want to scale while having guidance along the way.
Resend adjusts IPs dynamically, meaning the API will adjust to your sending volume in real-time. This makes Resend ideal if you have a fast-growing app with variable sending volumes or a website with traffic spikes.
Postmark distributes email traffic evenly to prevent delays and routes emails through the closest server to the recipient. Thanks to this, it’s considered a reliable choice if you plan to send transactional emails.
Mailgun offers its Rapid Fire Delivery SLA, an agreement promising safe and reliable delivery of up to 15 million emails per hour, delivered under certain pricing plans. This is ideal for senders who plan to perform email blasts.
SendGrid sends emails from servers closest to recipients and combines this with multiple dedicated IPs to ensure stability during spikes. This makes it best for teams who plan to send both marketing and transactional emails at a high volume.

IP infrastructure

No matter how great your content is or how clean your contact list is, your emails won’t land in the primary inbox if your ESP doesn’t have a solid IP infrastructure, which consists of dedicated IPs, features like auto warm-up, and more.

Here’s a quick breakdown for you:

Email service provider	IP infrastructure
Mailtrap	Dedicated IPs available from 100k emails/month with auto warm-up. On the Business 100k plan. High-quality shared IPs.
Resend	Dedicated IPs available as an add-on from 500 emails/day with auto warm-up. On Transactional Scale or Pro Marketing plans. Auto warm-up feature.
Postmark	Dedicated IPs available as an add-on from 300k emails/month. Managed and self-managed warm-up options. Shared IPs with good reputation.
Mailgun	Dedicated IPs available from 50k emails/month as an add-on and included from 100k emails/month. On the Foundation plan. Auto warm-up feature.
SendGrid	Dedicated IPs available from 100k emails/month. From Email API Pro or Marketing Campaigns Advanced plans. Auto warm-up feature.

What it means:

Mailtrap offers free dedicated IPs for senders with a volume higher than 100k/month, has automatic warm up and high-quality shared IPs. This makes it super flexible for teams who plan to scale gradually or send over 100k emails per month.
Resend provides dedicated IPS as an add-on under certain plans, making it perfect for teams who are just starting out and have a low sending volume.
Postmark offers dedicated IPs as an add-on if you’re sending 300k emails/month. It also offers options for managed or self-managed warm-up, which makes it a solid choice for experienced teams, especially smaller senders, since its shared IPs are very reputable.
Mailgun is slightly more flexible with dedicated IPs as an add-on for plans with more than 50k emails per month. It also includes auto warm-up, making it great for inexperienced teams and smaller senders.
SendGrid provides dedicated IPs for free if you’re sending more than 100k emails per month and has auto warm up. This makes it a great choice if you’re looking for a solid API to send marketing and transactional emails.

Email streams

If you plan to send mass emails or different types of emails, having separate sending streams can make a world of difference for your sender reputation. Nowadays, some providers offer a separate infrastructure through IP pooling and dedicated IPs, whereas some also offer a true separate stream.

Here’s a super quick breakdown:

Email service provider	Email streams
Mailtrap	✅ Bulk and transactional
Resend	❌
Postmark	✅Transactional and broadcast
Mailgun	❌
SendGrid	❌

As you can see, only Mailtrap and Postmark have separate sending streams. You can also configure other APIs to do a similar job, but it won’t be as efficient as a true separate stream.

Multitenancy support

If you have several different teams, clients, domains, etc., you’ll need an email API with high-level multitenancy support. Most providers offer multitenancy via subaccounts or sub-users, but some also provide it via servers or domains.

Email service provider	Multitenancy support
Mailtrap	✅ 100–3,000 domains depending on plan
Resend	Multiple domains, unclear if multi-tenant; 10–1,000 for paid plans, flexible for enterprise
Postmark	✅Through servers. 5–10 servers and domains. Unlimited for Platform plan
Mailgun	Subaccounts for enterprise. 1–1,000 domains depending on plan
SendGrid	Sub-user accounts. Up to 3,000 domains per user

Based on the table:

Mailtrap is best for SaaS platforms or agencies with many clients.
Resend is best for teams who don’t need true multitenancy.
Postmark is best for teams that need to separate clients and apps.
Mailgun is best for those who want to separate tenants between units, clients, or regions.
SendGrid is best for teams with many sender profiles.

I hope you enjoyed reading our in-depth comparison of the best email API providers. This article includes selected highlights and was originally published in full on the Mailtrap Blog, which you can visit to read the complete guide.

Which Email API Offers The Most Flexibility: In-Depth Comparison of Best Providers