KumoMTA has announced its April 2026 “Astronomical Spring” release, introducing a series of operational and developer-focused improvements for large-scale senders.
The headline addition is a new kumo.jsonl module for reading and writing zstd-compressed JSONL log segment files. This enables operators to ship logs and run webhook processing out of band, using separate long-running processes with their own checkpointing, retry handling and failure modes, rather than tying that work directly into the MTA hot path.
The release also improves MIME handling for non-conforming messages, including cases where binary content such as Shift_JIS appears without correct MIME encoding declarations. This should help operators relaying international mail while giving Lua policy authors more control over whether to preserve or rewrite non-conforming content.
Other notable updates include per-source DNS resolution strategy via a new ip_lookup_strategy option, clearer summary errors for proxy and unplumbed source address failures, and a DKIM verification change: From-domain mismatch no longer automatically creates a policy failure.
A number of smaller quality-of-life improvements are also included, covering header parsing resilience, queue helper startup checks, deterministic JSON output, new Lua helper functions, HTTP injection templating updates and SMTP server fixes.
The release is available now via KumoMTA package repositories and Docker containers. KumoMTA recommends users review the full changelog, particularly if they rely on automation or Lua policy affected by the DKIM, MIME or connection error handling changes.
