Lettermint | builds scalable European email infrastructure with KumoMTA and Postmastery Console
"After spending weeks trying to make Postfix work the way we needed, we switched to Kumo and had a functional API and SMTP relay implementation running in a single day."
Building a sovereign European email platform
Lettermint is a Netherlands-based email service provider focused on delivering reliable, privacy-first email infrastructure fully hosted within the European Union. Development started in late 2024, with the platform officially launching in May 2025. The company was founded with a clear mission: provide sovereign European email infrastructure with production systems hosted in Amsterdam and backup infrastructure in Germany.
Initially focused on transactional email for freelancers and small agencies, Lettermint quickly expanded into broadcast email services and now supports a diverse customer base including e-learning platforms, political organizations, e-commerce businesses, and public sector institutions such as the Province of North Holland. The company provides API and SMTP-based email delivery infrastructure focused on reliability, privacy, GDPR compliance, and sovereign European hosting.
As customer volume and sending complexity increased, the team needed a scalable approach to deliverability monitoring, bounce processing, and infrastructure management.
The challenge
In the early stages, Lettermint managed bounce handling and spam complaints through manual log reviews. While workable at low volumes, this approach quickly became operationally expensive and difficult to scale. "At the beginning, manually reviewing logs was manageable. But as volume increased, it became clear we needed proper tooling and automation." The team initially attempted to build their infrastructure around Postfix but encountered significant operational complexity and outdated documentation, making long-term maintenance difficult.
At the same time, Lettermint needed:
- A modern MTA platform that could scale rapidly
- Better visibility into deliverability and reputation metrics
- Automated bounce and complaint handling
- Monitoring and alerting without constant manual log analysis
- An EU-based partner aligned with their sovereignty requirements
- An overall costs effective solution
Selecting KumoMTA and Postmastery
After evaluating available solutions, Lettermint selected KumoMTA together with Postmastery Console. One key factor was the strong alignment between the platforms and the ability to integrate quickly without extensive custom engineering. The migration from Postfix to KumoMTA proved dramatically faster than expected.
"After spending weeks trying to make Postfix work the way we needed, we switched to Kumo and had a functional API and SMTP relay implementation running in a single day."
KumoMTA provided the modern architecture and automation capabilities the team needed, including shaping automation, built-in SMTP handling, and webhook integrations for processing out-of-band bounces and spam complaints. At the same time, Postmastery Console gave the team centralized visibility into delivery performance, sender reputation, tenant activity, infrastructure health at an extremely competitive price point.
Operational visibility and faster troubleshooting
As Lettermint scaled, operational visibility became critical - especially while operating with a limited number of IP addresses.
Postmastery Console enabled the team to quickly identify which tenants caused bounce spikes, reputation issues, or unusual traffic patterns. This visibility helped distinguish legitimate content problems from false positives and allowed issues to be resolved significantly faster.
The console also became essential for monitoring IP and domain reputation.
"One false-positive Spamhaus listing can create major operational pressure when you only have a small IP pool. Postmastery helped us detect and mitigate those situations very quickly."
The combination of KumoMTA and Postmastery Console replaced repetitive manual monitoring workflows with automated alerting and centralized analytics. Instead of checking logs multiple times daily, the engineering team now relies on proactive alerts and drill-down reporting.
"That shift alone saved us significant engineering time every day and allowed us to focus on platform growth instead of constant operational monitoring."
Lettermint describes the integration between KumoMTA and Postmastery as highly seamless.
"The mapping between Kumo and Postmastery works extremely well. There’s very little configuration required, and it saves hours of manual log analysis."
A collaborative partnership
Beyond the technology itself, Lettermint highlighted the responsiveness and collaboration from the Postmastery team. This included proactive assistance with tenant mapping improvements, operational guidance, and rapid implementation of requested functionality such as Single Sign-On (SSO).
The relationship has evolved beyond infrastructure support into ongoing collaboration around future observability and analytics capabilities.
Looking ahead
As Lettermint continues to expand its European email platform, the company is exploring additional opportunities to enhance operational intelligence through AI-assisted analysis and observability tooling.
Future ideas discussed with Postmastery include integrating Large Language Models (LLMs) into operational workflows to accelerate debugging, correlate infrastructure metrics, and identify problematic tenant behavior faster.
With KumoMTA and Postmastery Console in place, Lettermint has established a scalable and cost effective operational foundation that allows the team to focus on growth, deliverability, and customer reliability rather than infrastructure maintenance.
About Postmastery
Postmastery helps legitimate senders be recognised as trusted senders and reach the inbox. Our client base includes email service providers (ESPs), marketing agencies, and demanding professional senders. Postmastery is a global partner of leading MTA vendors - PowerMTA, GreenArrow, Halon, KumoMTA, Postfix - and offers integration services, local support, and deliverability optimisation.