Facing a surge of European demand for transactional email that never touches US cloud infrastructure, Dutch provider Lettermint built its entire sending stack on KumoMTA from day one, growing from a side project to 200,000 emails a day while staying fully GDPR-sovereign
Lettermint is a Netherlands-based transactional email service provider built around a single, uncompromising premise: sovereign infrastructure. Every part of the Lettermint stack — compute, storage, IP ranges, and support — runs on European-owned infrastructure, with no dependency on AWS, Google Cloud, Microsoft Azure, or any other US-headquartered cloud provider. For the growing number of European organizations that want GDPR compliance and, increasingly, complete independence from US corporate and legal jurisdiction, Lettermint occupies a deliberately narrow, defensible niche.
Founded by Bjarn Bronsveld, a DevOps engineer who took on the technical build, and Bjorn Antonissen, who concentrated on the initial UI/UX design, Lettermint went from an intriguing yet rudimentary tool popular with the developer community to a production platform handling hundreds of thousands of emails a day in a little over a year.
The idea for Lettermint started, as Bronsveld tells it, as something of a vanity coding exercise. In spring 2024, developer communities were buzzing about recent merger activity in the email services sector, where American entities were acquiring European ESP brands. Someone in the community floated the obvious provocation: why not just build a better, Euro-centric alternative? Bronsveld had relevant experience — he'd been running a monitoring platform and doing DevOps work — but treated the idea as a distant possibility rather than a plan.
That changed after the November 2024 US presidential election. As Bronsveld describes it, the days around the election and the January 2025 inauguration triggered a visible shift in sentiment among European developers and businesses, with a noticeable uptick in public discussion — much of it on LinkedIn — about the need for EU-based alternatives to American cloud and SaaS platforms. As Bronsveld explains:
"People started to, I don't really want to call it panic, but there was a bit of that energy on LinkedIn. People were looking for EU alternatives, and that actually intensified after the inauguration. More and more companies were looking for EU alternatives."
Bronsveld and Antonissen originally connected through a local startup meetup, and the two began building Lettermint in earnest in November 2024. By January 2025, they had arranged their own network and IP ranges, making a deliberate decision to avoid becoming dependent on any third parties for sending infrastructure or reputation. It's a decision that continues to sit at the center of what makes Lettermint's sovereignty claim credible rather than cosmetic: instead of simply hosting a rented mailbox-sending service inside an EU data center, Lettermint controls its own IP space and infrastructure end to end.
That control came at a cost. Initially, the Lettermint team chose to run a multi-tenant sending platform on Postfix, the once-standard but increasingly antiquated MTA platform. Because of the decision the pair quickly hit the limits of what a general-purpose MTA could do. Sorting bounce and complaint feedback by receiving provider, by sending domain, and by individual tenant — the basic plumbing any serious ESP needs — became an escalating engineering burden almost immediately.
"We got the sending to work, but that was the easy part. We had to know every response, every queue. We had to triage emails based on the provider we're sending to, based on the domain, based on which tenant is sending their email. And that started to become an issue."
Bronsveld and Antonissen evaluated some established options next: GreenArrow and PowerMTA, the two platforms he describes as "what we thought all the big guys were using." But as a bootstrapped two-person startup, they ran into the same wall many small ESPs hit with legacy commercial MTAs: licensing fees that are cost prohibitive for early-stage businesses.
Bronsveld doesn't recall exactly how he first came across KumoMTA, but the documentation is what convinced him it was worth a serious look.
"I think this is honestly the best documentation I've ever read. I've gone through documentation for other pieces of software back in the day, before AI could summarize it for you, so you had to read it yourself. The Kumo documentation was just really understandable, even as an engineer coming in from outside."
After reaching out through KumoMTA's Discord community, Bronsveld and Antonissen set aside a single day dedicated to getting KumoMTA integrated with the SMTP relay and API layer they'd already built for the admission and processing side of the platform. It was a long day, but by the end of it they'd delivered their first email through KumoMTA. Bounce processing, spam-complaint handling, and feedback-loop integration followed quickly after.
"From the day we started implementing KumoMTA, it went really quickly. After that, basing our platform on Kumo allowed us to focus on building out the platform itself, and not on worrying about the actual delivery infrastructure."
Since the team wanted full infrastructure control rather than a managed SaaS layer, Lettermint chose to run KumoMTA on Kubernetes, thereby establishing a self-managed deployment from day one. Because the sovereignty requirement ruled out AWS and the other hyperscalers' bring-your-own-IP options, the team's choices for EU-based infrastructure providers that would support their own announced IP ranges were limited. Lettermint started on OVHcloud's managed Kubernetes service, then moved to a dedicated private cloud with UpCloud, a Finnish provider with a datacenter in Amsterdam.
As the customer base expanded, Lettermint began using KumoMTA's Lua scripting layer to manage per-customer IP pool assignment directly, tagging outbound messages by customer pool in the header and having KumoMTA's policy engine route each customer's traffic through the correct pool of IP addresses. That capability, implemented in fall 2025, gave Lettermint the ability to isolate customer reputations from one another and spread risk across multiple IPs as the platform scaled. This approach enabled the team to implement the same kind of granular, code-based traffic control that larger ESPs rely on KumoMTA for.
"The flexibility that Kumo provides with its Lua scripting definitely helped us later on. We needed to look into solutions to spread customers out over multiple IPs in case of issues, and being able to script that ourselves made it possible."
Lettermint launched publicly at the end of May 2025. Early customers were small, including freelancers and small marketing and communications shops sending a few thousand emails a month. By September 2025, three months after launch, the platform was averaging roughly 50,000 emails a month across a couple dozen small customers.
From there, growth accelerated sharply as larger and more diverse organizations began adopting the platform. Most of them were explicitly motivated by the sovereignty requirement. Lettermint's customer base now includes a German energy supplier, a government organization with dedicated email-delivery needs, several Dutch e-commerce businesses, and Volt Europa, the pan-European political party active across multiple EU countries. Lettermint has also begun working with Euro-Office, a company building a sovereign, EU-hosted alternative to Google Workspace and Microsoft 365.
As volume scaled, however, so did the operational burden of monitoring deliverability. This was a burden that had been manageable when Lettermint was sending a few thousand emails a month, but became untenable as complaint and bounce volumes grew in absolute terms. In the same autumn of 2025 timeframe, Lettermint began working with Postmastery, the European email consultancy. The Postmastery team led the effort to implement dedicated deliverability monitoring, dashboards, and alerting, layered on top of the KumoMTA sending infrastructure.
Today, Lettermint sends roughly 200,000 emails a day, up from 50,000 a month just nine months earlier. Throughout that growth, Bronsveld says KumoMTA has never been a bottleneck in the delivery chain. Lettermint publishes a public, continuously updated "time to inbox" metrics page — an idea Bronsveld borrowed directly from Postmark's own public metrics — that shows the end-to-end time from message admission to inbox delivery across major providers.
"If it takes five seconds to deliver an email to Gmail, KumoMTA accounts for less than a second of that. We're still working to optimize the rest of that pipeline, but we're pretty happy with how far it's gotten."
That level of performance and transparency has become a genuine sales asset for Lettermint, not just a vanity engineering metric.
"The documented speed metrics definitely helped. Whenever we're on a sales call or get a question, we can point to our public inbox metrics page. And even if there's an issue going on, we just show it. We're not trying to hide anything. It also helps us stay accountable for anything that comes up."
Lettermint continues to expand its use of KumoMTA's platform: in February 2026, the team moved from SMTP-based message admission to HTTP injection, and has also adopted the KumoMTA proxy. Looking ahead through the rest of 2026 and beyond, Bronsveld says the team's primary focus is improving load balancing across the proxy layer as volumes continue to climb.
Support, notably, has come almost entirely through KumoMTA's Discord community, including one bug that Bronsveld reported and was resolved quickly by the Kumo engineering team. For a two-person, bootstrapped startup building sovereign infrastructure from the ground up, that combination of open-source flexibility, a responsive community, and zero licensing overhead has been foundational to Lettermint's ability to compete on both principle and performance.
Europe's sovereign IT movement
Lettermint's founding story is a small, concrete example of a much larger shift underway across European technology procurement: a move away from treating US hyperscaler infrastructure as the default standard, and toward actively selecting EU-owned, EU-governed alternatives.
The legal root of the issue is the US CLOUD Act, which permits US authorities to compel US-headquartered companies to turn over data they control, regardless of where that data physically sits. That means a European company using a US provider's Frankfurt or Dublin data center has not necessarily escaped US jurisdiction over its data; the location of the servers doesn't change which country's courts can compel disclosure. This distinction between data residency (where data sits) and data sovereignty (whose laws actually govern it) has become the central technical and legal argument driving the sovereign IT movement.
That argument existed well before 2024, rooted in GDPR-era compliance concerns dating back to the first Trump administration and the 2020 "Schrems II" court ruling that struck down the previous EU-US data transfer framework. But sentiment has hardened considerably since the 2024 US election, with European businesses and public institutions increasingly framing the issue not just as a compliance question but as a matter of reducing exposure to a US political and legal environment many now view as unpredictable or adversarial to European interests.