KumoMTA published its Summer Solstice release (version 2026.06.23) on 24 June. Kumo calls it a small maintenance release with no breaking changes. It carries several fixes that matter for mail-ops.
Kumo describes the release as a workbench-clearing exercise before larger summer projects, and the headline point for operators is that there are no breaking changes. Existing deployments upgrade cleanly. What follows is a set of targeted fixes rather than new features.
The change most relevant to deliverability teams is in Traffic Shaping Automation. The shaping.lua helper’s setup_with_automation now takes optional skip_log_record_types and additional_skip_log_record_types tables, giving operators control over which log record types are published to TSA for evaluation. One replaces the default set, the other extends it. For anyone tuning how their shaping automation reacts to remote server feedback, that is a useful lever.
The release also fixes a STARTTLS problem with internationalised domains. When the SMTP client tried to negotiate STARTTLS to an IDNA domain written in its Unicode form, it could fail with a “not a valid DNS name” error, for example on a domain such as münchen.de. Mail to those domains now negotiates TLS correctly.
Two fixes touch feedback and logging. Feedback log records produced for incoming ARF reports were serialising the original message as an array of byte values instead of the documented string, which is now corrected for UTF-8 content. Separately, the JSONL log tailers no longer shut down when handed a truncated or invalid file. The offending file is logged and skipped, and remembered so it is not retried.
There is also a fix to kumo.crypto.aws_sign_v4, the AWS SigV4 signing helper, which corrects inverted x-amz-content-sha256 handling and tightens header canonicalisation against the official AWS test vectors. One thing to note if you sign requests for non-S3 services such as SNS or SQS: you may now need to pass x-amz-content-sha256 explicitly, or stop sending it, to match the signed request.
The full changelog has the detail, and the release is available through Kumo’s package repos and Docker container.
Disclosure: KumoMTA is an Enterprise Member of Emailexpert. Coverage decisions are made independently of commercial relationships.