We migrated our email infrastructure to KumoMTA. Here's why it matters for you.

If you've been sending email through Postmark lately, your messages have been moving through an entirely new engine (and you probably didn't notice a thing). That's exactly how we wanted it.

We've completed our migration from PowerMTA to KumoMTA, an open-source, high-performance mail transfer agent (MTA). Every email Postmark sends now runs through KumoMTA. It's the biggest infrastructure change we've made in years, and it sets the stage for everything we want to build next.

Wait, what's an MTA? #

An MTA,or mail transfer agent,is the software responsible for delivering your email. When you send a message through Postmark's API, our application processes it, and the MTA takes it from there: queuing, connecting to the recipient's mail server, and handling the SMTP transaction that gets your message where it needs to go.

Think of it as the delivery truck for your email. You hand us the package, and the MTA figures out the fastest route to the door.

Why we made the switch #

Postmark has been running PowerMTA for years, and it served us well. But as our needs evolved, the limitations became harder to work around.

The biggest issue? Traffic management. Our previous setup made it difficult for our deliverability and operations teams to respond in real time when something went sideways, like when a mailbox provider temporarily blocks an IP address. In those moments, you need to be able to back off traffic, reroute, and adjust shaping on the fly. Our old configuration didn't give us that flexibility at the MTA level, which meant we were sometimes limited in how quickly we could resolve delivery issues.

Beyond that, our previous infrastructure was running on aging hardware that was increasingly difficult to maintain. Scaling required a lot of manual effort, and customizing behavior to match Postmark's specific needs was constrained by the proprietary nature of the platform.

We needed something that could grow with us, and that we could shape to fit the way Postmark works, not the other way around.

Why KumoMTA #

KumoMTA is an open-source MTA built by a team of email infrastructure veterans for high-volume sending environments. It's written in Rust, released under the Apache 2 license, and designed from the ground up for the kind of work Postmark does.

Here's what made it the right fit:

Scalability without limits. KumoMTA runs on modern cloud infrastructure with auto-scaling. No more capacity ceilings tied to specific hardware. We scale up and down based on what you need.

Full control over traffic shaping. This is the big one. KumoMTA gives our deliverability team granular control over how traffic flows to every major mailbox provider. When Gmail, Yahoo, Microsoft, or Apple need us to adjust our sending patterns, we can respond immediately, not hours later. That means fewer deferred messages and faster inbox delivery for your recipients.

Real-time observability. KumoMTA delivers logs and events back to our application in real time via webhooks. This feeds directly into the delivery data you see in your Postmark dashboard and improves how quickly we can diagnose and resolve issues.

Customization that fits our model. Postmark isn't a typical bulk sender. We run a multi-tenant environment where every customer's reputation matters. KumoMTA's Lua-based configuration lets us tailor behavior precisely to our needs: from how we handle bounces to how we manage IP pools across Streams.

Open source, open road. With an Apache 2 license and source code on GitHub, KumoMTA gives us full visibility into the technology that powers your email delivery. No black boxes, no licensing surprises, and a community of some of the largest senders in the world contributing to make it better.

What the numbers say #

We don't make infrastructure changes based on gut feeling. We tested KumoMTA extensively before fully migrating over, and the results confirmed what we were hoping for.

Average queue time: the time between Postmark accepting your message and completing the SMTP handoff to the recipient's mail server has improved across every major mailbox provider: Queue time is analogous to message delivery time, faster queue time = faster delivery.

• Gmail: ~1.2 seconds with KumoMTA, down from ~2 seconds
• Yahoo: ~3.2 seconds with KumoMTA, down from ~4.6 seconds
• Microsoft: ~2.8 seconds with KumoMTA, down from ~4.8 seconds
• Apple: ~6.3 seconds with KumoMTA, down from ~8 seconds

Bounce rates held steady or improved slightly across providers, which tells us the migration maintained the sending reputation we've spent 16 years building (while delivering faster).

In nearly every scenario, KumoMTA is delivering as fast or faster than our previous setup. And because we now have much finer control over traffic shaping, we're better positioned to keep it that way as mailbox providers continue evolving their requirements.

What this means for you #

Here's the short version: your emails get delivered faster, and we're better equipped to keep them out of trouble.

More specifically:

Faster delivery. The queue time improvements above translate directly to your recipients getting password resets, order confirmations, magic links, and notifications sooner. In transactional email, every second matters.

Better deliverability resilience. When a mailbox provider tightens its throttling or temporarily blocks an IP, our team can now adjust traffic in real time. That means fewer disruptions to your delivery and faster recovery when issues come up.

A more modern, reliable foundation. Our MTA infrastructure now runs on modern cloud infrastructure with auto-scaling, comprehensive monitoring, and a full suite of end-to-end tests. This is a meaningful step up in operational reliability.

Room to grow. This migration isn't just about today, it's about what comes next. KumoMTA's flexibility gives us a foundation to build features and optimizations that weren't possible before.

Transparent about the trade-offs #

If you look at the bounce rate data, you'll notice Apple's rates are higher than other providers for both the old and new systems. That's not a KumoMTA issue — Apple's mail infrastructure behaves differently from other major providers. Bounce rates there reflect Apple's unique handling of certain message types, and that's consistent regardless of MTA.

We're sharing this because we'd rather give you the full picture than cherry-pick the numbers that look the best. 

An investment in what matters

Postmark has always been about one thing: making sure your emails reach real people, fast and reliably. This migration to KumoMTA is a significant investment in that mission.

You shouldn't have to think about the infrastructure behind your transactional email, you've got your own product to build. But when you do look under the hood, we want you to see a team that's constantly investing in speed, deliverability, and reliability.

Your emails are in good hands. Faster ones, now.

Have questions about our infrastructure or how this affects your account? Contact our support team — we're always happy to nerd out about email delivery.