It's release time again!
Seems like we just did this a few weeks ago - Oh right, WE DID! Thirteen weeks ago, to be exact, which is actually a longer break than we would normally have, but it was summer, and there is a lot packed in here. Let me tell you, this one was an adventure. There are many contributions from the community, along with some significant control and visibility changes.
The fifth stable release of 2026 [Release 2026.09.22-a276d4a8] was built and shipped on September 22nd, followed by a hardening release on the 29th with additional security enhancements. That hardened, stable release [Release 2026.09.29-b90d8bc1] is now available for public access at all your favourite locations. As usual, you can clone the repo, install with APT, YUM, or DNF package managers, or grab the latest Docker image.
As DANE adoption grows in the community, we needed changes and more flexibility. This release includes at least five DANE configuration changes, highlighted by the ability to use the embedded Hickory resolver, saving users from having to install and manage a separate Unbound resolver.
This release includes many spool management and reporting functions to improve control and visibility of the core message spool. Measuring rocks_spool_actual_delayed_write_rate might not be interesting for most people, but when you are pushing a billion messages a day, this can be valuable data. We also added queue inspection facilities to give users more visibility into actual message data.
As if message control functionality weren't enough, we've added several new Lua functions that allow for pre-calculation of queue and path constraints. These allow a mailops engineer to evaluate a running configuration on-the-fly.
And as always, we are grateful to our user community for their valuable contributions.
- @Solmea was kind enough to provide updates to QQ, 163, and Yahoo Japan as well as additions for NetNavigator and Barracuda.
- @dschaaff suggested a memory management enhancement that helps prevent a phantom cache pressure issue.
- @kayozaki suggested a change to Base64 MIME body handling that adds compatibility with other mail software.
- @alexanderwburch suggested a mod to allow for stray spaces in the envelope. Not strictly RFC-compliant, but helpful if you have a non-RFC-compliant generator.
As I mentioned, this release includes many changes, including a few important breaking changes, so if you manage your KumoMTA deployment, this is worth a careful read before deploying.
You might also notice that we are publishing a blog post announcement a full week after the periodic stable release. This has everything to do with security hardening and providing our paid sponsors a few days of lead time to upgrade systems. Now we are notifying everyone else that the hardened, stable release 2026.09.29-b90d8bc1 is now available for download, and we recommend upgrading to it at your earliest convenience.
If you are loving KumoMTA, we want to hear from you. Tell us if you have any suggestions for the next release.
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KumoMTA is the first open-source MTA designed from the ground up for the world's largest commercial senders. We are fueled by Professional Services and Sponsorship revenue.
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