tl,dr: KumoMTA is not just Open-Source, it's also the best MTA on the planet for large-scale senders.
One thing I've noticed when we tell people KumoMTA is Open-Source is we get a mixed response: those who understand exactly why an Open-Source MTA is important to their business treat it like it's kind of a big deal, those people who associate Open-Source with being more limited than commercial software (despite a myriad of Open-Source applications which are in use at enterprises of all sizes such as Apache, MySQL, Postgres, Nagios, and Prometheus).
Open Source, and Commercial-Grade
I want to make one thing clear: KumoMTA is enterprise-grade and we certainly could have made it a commercially licensed product if we had chosen to, because like its commercial cousins it meets or exceeds the needs of the largest senders in the world:
Performance - KumoMTA is FAST. We're talking millions and millions of messages per hour on proper hardware (see the earlier link for examples), enough to handle the vertical scaling needs of the largest senders in the world. Even with all that speed, we understood early on that many of those senders wanted also to scale horizontally, which is why we publish a pre-built Docker container for Kubernetes and container users, leverage Redis for shared throttle counters across clustered nodes, and support both HAPROXY and SOCKS5 proxies to ensure that clustered nodes can operate as a cohesive group.
Compatability - KumoMTA is pre-built on five different modern operating systems, and if your OS of choice isn't on the list, we've test-built it on over a dozen other operating systems as well as other architectures such as ARM.
Security - Whether it's modern security for securing the SMTP session such as TLS 1.3, MTA-STS, and DANE, or the use of secure vaults for DKIM keys and SMTP AUTH credentials, KumoMTA brings the latest standards and practices in securing your email channel. In addition, unlike almost all current commercial and Open-Source MTAs that are built on C/C++, our KumoMTA codebase is written 100% in Rust; a modern language with built-in memory safety that was recently recommended over C/C++ by the US White House Office of the National Cyber Director to help improve national security around critical IT infrastructure and in an NSA cyber security report from 2022.
Flexibility - KumoMTA goes beyond existing solutions with static configs that sometimes have scripting or Milter capabilities. Instead, it uses the Configuration as Code model, enabling MailOps administrators to accommodate almost any conceivable use case when integrating with modern DevOps environments. Where traditional configuration integration requires setting up tooling to gather configuration from a data source and write it to proprietary config file formats, KumoMTA administrators can implement their configuration using Lua hooks that can speak directly to those data sources if desired.
In addition, because the configuration is code, KumoMTA doesn't need to be restarted or manually refreshed when the configuration changes: KumoMTA only loads into memory the configuration needed for the active sending tenants, domains, and IPs, caching the configuration for a few minutes then re-reading from the source. Most configuration changes just require a short wait until the cache refreshes.
Integration - In addition to the integration benefits that come from Config as Code, KumoMTA is designed to be an integrated component of a modern MailOps environment. While we support traditional logging to disk, we also support the delivery of log data (and even email messages) via HTTP Webhooks, AMQP, and Kafka. We also provide an extensive and growing set of APIs for tasks ranging from server administration to monitoring metrics (in both Prometheus and JSON formats).
Support - As you can tell from all the links, we have extensive documentation (roughly 300 pages at the time of writing). In addition, we have a growing Discord community of users providing mutual support and assistance, For Enterprise users we provide extensive Professional Services and Support offerings, including 24/7 SLAs for those who want extra peace of mind.
Commercial-Grade, and Open-Source
As you have gathered by now, KumoMTA meets or exceeds the capabilities expected of commercially licensed MTAs. While we could have sold it under a commercial license, we chose to release it as Open-Source. Why? Because we firmly believe that Open-Source is better for our customers and users.
Over the years we've seen commercial MTA users get run through the wringer as vendors try and squeeze every last dollar to satisfy the desire of their boards for "hockey-stick growth" and to position for acquisition so that the VCs can take their money off the table. Some large senders have been so burned they even turned to building their own internal MTAs, determined to never again be beholden to a vendor, paying year after year for support & maintenance to ensure their MTA infrastructure doesn't stop working or get woefully out of date in a world of evolving deliverability and security requirements.
We've been in this space a long time (our shortest tenure in the email space is fifteen years) and we know from experience that large-scale senders not only need an enterprise-grade MTA, they need infrastructure they can trust. KumoMTA is secure, stable, and provided by a vendor senders can trust not to change terms and prices come renewal time after they've become dependent on their MTA for their business-critical messaging needs because our use of an Open-Source license makes it impossible for us to cut our users off now or in the future.
Would you like to free yourself from outdated and insecure MTA infrastructure from a vendor more interested in your money than your success? Contact us and let's talk about how you can adopt the last MTA you'll ever need.