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):
- Infrastructure and scalability
- Sending logic and performance
- Integration and extensibility
- Templates and personalization
- Testing environment
- Customer support
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 |
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.
Sending logic and performance
Don’t worry, I didn’t just consider the sending limit of each API. I took into consideration the following:
- Rate limits and throttling
- Queueing
- Retry logic
- Batch API
These elements not only make an email API flexible, but they also ensure stable delivery by not overwhelming receiving servers, meaning your emails are more likely to reach the main inbox.
Rate limits and throttling
Rate limits define how many requests an email API can make over a set period (e.g., 1,000 requests per second), whereas throttling controls the request speed. In translation, both give you control over your sending.
And here’s what the ESPs offer in this regard:
| Email service provider | Rate limits and throttling |
| Mailtrap | No limits. Customizable throttling |
| Resend | 2 requests per second. No information on throttling logic |
| Postmark | No limits. Automatic throttling |
| Mailgun | Yes, but not specified |
| SendGrid | 10,000 requests per second, up to 1,000 recipients per email |
Quick breakdown:
- Mailtrap has no limits and lets you customize throttling, making it ideal if you want to fine-tune your sending behavior.
- Resend has enforced 2 requests per second and doesn’t publicly state if throttling is customizable, so it’s best for teams who don’t need flexibility or control when it comes to sending.
- Postmark has no rate limit, but it handles throttling automatically, so it’s great if you’re looking for a hands-off sending control.
- Mailgun doesn’t state its enforced rate limits, but you can increase it by contacting their support.
- SendGrid has a super high sending throughput and recipients per email, making it great for high-volume senders.
Queueing
Sending a large volume of email at once is never a good idea, so queueing them is a must if you don’t want to end up in spam. Most ESPs do it automatically, but some also allow you to do it within their APIs or manage it manually.
Check it out:
| Email service provider | Queueing |
| Mailtrap | Automatic. Can also be managed manually |
| Resend | Automatic within Batch API |
| Postmark | Automatic |
| Mailgun | Automatic. Can also be managed manually |
| SendGrid | Automatic. Can also be managed manually |
You should go with:
- Manual queueing if you plan to send large volumes of email or time-sensitive messages, as you’ll need to tailor queues by recipient domain, app environment etc.
- Automatic queueing if you want a minimal setup and don’t plan to send bulk email.
Retry logic
Sometimes, you can do everything right, but emails will fail to send due to uncontrollable issues like recipients’ servers being unavailable, DNS hiccups, etc. In this case, you need an email API that can handle these failures with retry logic, which varies from provider to provider, mostly based on its retry time window. Check it out:
| Email service provider | Retry logic |
| Mailtrap | For 24 hours |
| Resend | Managed by the user |
| Postmark | For 12 hours every 10 minutes per domain |
| Mailgun | For 8 hours |
| SendGrid | For 72 hours |
Tip: If you plan to send sensitive emails like payment confirmations, go with an ESP that has long retry windows (e.g., Mailtrap or SendGrid).
Batch API
Sending emails in groups, or batches, is an industry-standard practice that takes an unnecessary load off your email infrastructure. Now, each provider offers different batching loads, so you can narrow down your search based on your batching limit preferences:
| Email service provider | Batch API |
| Mailtrap | 500 messages per API call. Payload size up to 50 MB, including attachments |
| Resend | 100 emails per API call |
| Postmark | 500 messages per API call. Payload size up to 50 MB, including attachments |
| Mailgun | Up to 1,000 recipients per API call |
| SendGrid | Different Batch logic, no specific limits |
Tip: If you plan to send the same email (e.g., newsletter or product update) to many recipients at once, providers like Mailgun or Postmark are a great choice since they have high-volume batch calls. You can also track each group separately with SendGrid’s Batch ID logic, which is the most flexible option here.
Integration and extensibility
In the following section, I’ll focus on ease of integration and control you get over each email API by covering:
- Supported SDKs and third-party integrations
- Webhooks and supported events
SMTP integration & SDKs
To allow users to quickly and easily set up their sending functionality, most ESPs offer SDKs (some with ready-made code snippets) for major programming languages. For advanced workflows, there are third-party integrations.
However, as you might have guessed it, the selection of SDKs and integrations varies greatly, so here’s a breakdown:
| Email service provider | SDKs | Third-party integrations |
| Mailtrap | Node.js, PHP, Ruby, Python, Elixir, Java | Zapier, Make.com (coming soon), n8n.io |
| Resend | Node.js, PHP, Laravel, Python, Ruby, Go, Java, Rust, .NET | AI tools, no-code tools, notifications tools, developer tools |
| Postmark | Ruby, Ruby on Rails, .NET, Java, PHP, Node.js | Email marketing platforms, CMS tools, email render and design tools, WordPress |
| Mailgun | Python, Go, Node.js, PHP, Java, Ruby | CRM, e-commerce, marketing, CMS, contact management, and other platforms |
| SendGrid | C#, Go, Java, Node.js, PHP, Python, Ruby | CRM, e-commerce, CMS, contact management, other platforms |
Table breakdown:
- Mailtrap offers a plethora of SDKs; however the only automation it offers currently is Zapier (more coming soon). Additionally, it can be integrated with Make, N8n.io, MCP agents, and Supabase, all of which are great no-code solutions.
- Resend has the most SDKs and can be connected with no-code tools, which means it’s an ideal choice for teams who are working with newer stacks and automation-heavy workflows.
- Postmark supports a variety of major programming languages as well as integrations with CMS, email design tools, and even marketing platforms. This makes it great for product and marketing teams.
- Mailgun has solid SDK support, but it truly starts to shine when it comes to integrations with CRMs, CMSs, e-commerce platforms, you name it. It’s a solid choice for large teams looking to connect to third-party apps.
- SendGrid has support for major programming languages as well as CRM and CMS platforms, making it great for e-commerce businesses.
Webhooks and supported events
Want to know how your emails perform in real-time? Webhooks allow you to do just that with instant notifications on key events like deliveries, bounces, unsubscribes, etc. They’re great not only for monitoring engagement, but also for debugging and triggering workflows in your system.
Most email APIs nowadays support webhooks, but the types of events you can track and retry logic vary greatly. Here’s what I mean:
| Email service provider | API for webhooks | Supported events | Retry logic |
| Mailtrap | ✅ | Delivery, bounce, unsubscribe, spam complaint, reject, soft bounce, suspension, open, and click + activity logs for enterprise | 40 retries every 5 minutes |
| Resend | ✅ | Email sent, delivered, delivery delayed, complained, bounced, opened, clicked; contact created, updated, deleted; domain created, updated, deleted | For 10 hours with a specific schedule |
| Postmark | ✅ | Delivery, bounce, spam complaint, open, link click, and subscription change | For 6 hours with a specific schedule |
| Mailgun | ✅ | Accepted, delivered, clicks, spam complaints, unsubscribes, permanent failures, and temporary failures | For 8 hours on a specific schedule |
| SendGrid | ✅ | Opened, clicked, unsubscribed, spam report, group unsubscribes, group resubscribes, processed, dropped, deferred, bounced, delivered | For 24 hours |
Webhooks breakdown:
- Mailtrap has a wide range of webhook events, and since it has 40 retries every 5 minutes, it’s a strong choice for teams who are looking for fast error handling. A bonus is that you can track all events separately per domain for each of the two Mailtrap streams.
- Resend webhooks support both email and contact/domain-level events, which makes it great for developers who, on top of emails, need control over their contacts.
- Postmark webhooks support various email events and changes per separate stream, making it great for people who want to send different types of transactional emails.
- Mailgun offers standard email engagement events, has solid retry logic, and can be configured at the domain level, making it a solid, flexible solution for teams with multiple domains.
- SendGrid has the longest webhook event list, although the downside is that the number of webhooks you can have is based on the pricing plan. But if you need detailed event tracking and long retry windows of up to 24 hours, SendGrid is a great choice.
Templates and personalization
A flexible email template lets you send personalized messages more easily via variables, streamlines collaboration with shared designs, and saves your team’s time.
So, here’s which email API has a template engine and the variables it uses:
| Email service provider | API for templates | Variables |
| Mailtrap | ✅ | ✅ handlebars engine |
| Resend | ❌ | ❌ |
| Postmark | ✅ | ✅ Mustachio engine |
| Mailgun | ✅ | ✅ Handlebars engine. Supports versioning |
| SendGrid | ✅ | ✅ Handlebars engine. Supports versioning |
Additional details:
- Mailtrap offers both a Templates API and drag-and-drop editors from its UI. Its SDKs support template sending, so it’s great for developers who want to automate personalized messages.
- Resend unfortunately doesn’t have a template engine, meaning you’ll have to create templates manually.
- Postmark has a templates API as well as an HTML editor in the UI. Its Mustachio engine supports dynamic content detection, complex keys, etc., which makes it ideal for advanced transnational email senders.
- Mailgun supports templates both via an API and the visual editor. Besides using the Handlebars engine, it also includes tags, which are great for those who want to perform A/B testing.
- SendGrid has a powerful template API with support for dynamic transactional emails, making it a perfect solution for teams with a complex email workflow.
Testing environment
Integrating an API into your app/project means tweaking and adjusting your app or project’s code a little bit. So, testing your sending functionality and emails before moving straight to production is key. That is, if you want to ensure your designs are rendered correctly, your domain isn’t getting blacklisted, etc.
A lot of API providers know this, so they offer a testing solution as well. Some provide a dedicated sandbox environment, while others only have test email addresses. The following table has all the testing details you need to know about:
| Email service provider | Sandbox environment | API for testing | Testing options | Separate product |
| Mailtrap | ✅ | ✅ | HTML/CSS validation, Spam Analysis, Blacklist Report, header and variable validation, template testing, testing automation, bounce emulator | ✅ Yes, 100 test emails/month for free for all plans |
| Resend | ❌ test email addresses only | Email sending API | Delivery, bounces, spam complaints | ❌ |
| Postmark | ✅ | ✅ | Email sending and delivery, testing API, testing webhooks, fake bounce generator | ❌ included in plans |
| Mailgun | ✅ | ✅ | Mailgun Inspect: accessibility testing, email previews, link and image validation.Mailgun Optimize: blacklist and spam trap monitoring, Google Postmaster and Microsoft SNDS, bounce classification, inbox placement, previews, alerts, email health score.Test mode | ✅Yes, two separate products |
| SendGrid | ✅ | ✅ | Inbox rendering test, spam test, link validation test + Sandbox mode | ❌available from Email API Pro plan and for all plans in Marketing Campaigns. Available credits vary based on plan |
Table breakdown:
- Mailtrap offers a full-fledged sandbox environment with design preview, spam and blacklist checks, API access, automated test flows, and more. This makes it ideal for teams who want no surprises in production and who wouldn’t say no to automating QA.
- Resend only offers test email addresses and some reports on bounces and spam complaints, which makes it perfect for lightweight testing and new teams who are looking for basic delivery confirmation.
- Postmark includes an almost complete testing environment. I say almost complete because it doesn’t support spam or HTML validation. This makes it a good choice for people looking to send only basic transactional emails.
- Mailgun offers two standalone suites: Inspect and Optimize, which you can use for complete email testing from A to B. However, they’re both separate paid solutions, so that makes them great for bigger teams or those who have a few bucks to spare.
- SendGrid offers a sandbox environment, spam analysis, and design preview, but only on higher-tier plans. This makes it a good option if you’re already on those plans.
Customer support
Last but not least, I’ve gone over the customer support options and availability each email API provider offers, since you can never know when things might go south.
So, here’s how and when you can reach each provider’s customer support:
| Email service provider | Support type | Availability |
| Mailtrap | Ticket, chat, priority support | 24/7 |
| Resend | Ticket and Slack | 1 PM to 12 AM GMT, Monday through Friday |
| Postmark | Ticket | 3 AM to 7 PM EST, Monday through Friday |
| Mailgun | Ticket, chat, and phone | N/A |
| SendGrid | Ticket, chat, and phone | 9 AM to 6 PM US Pacific, Monday through Friday. 24/7 for some plans |
Note: The support type and availability mostly depend on the pricing plan you choose. For example, Mailtrap has 24/7 customer support via tickets available to all users, even on the free plan, whereas you need to be a paying user to get support from Mailgun.
Wrapping up
Choosing an email API for you or your team is like choosing your tech stack: get it right, and you’ve got yourself a smooth workflow. Choose the one wrong, suffer a headache trying to land in recipients’ inboxes.
Email API flexibility plays a big role in avoiding this headache, so I hope this article brings you one step closer to making the correct decision.
Further reading:
- Best Email Service Providers and Services Tested and Reviewed
- SMTP Providers Compared: Choose the Best One for Your Business
- I Compared 7 Best Transactional Email Services: Here’s What I Found
- How to Choose Email Service Provider: Actionable Q&A Guide