KumoMTA’s latest release (2025.10.06-5ec871ab) introduces an inline MIME parser that lets operators decompose messages into their component parts – TEXT, HTML, attachments – and modify them on the fly.
Why it matters: High-volume senders can now handle security scanning and message modifications directly in the MTA without bolting on third-party tools.
Practical applications:
The KumoMTA team highlights several use cases: scanning outbound messages for credit card numbers and redacting them before delivery, extracting and quarantining suspicious attachments with automatic subject line modification, ensuring TEXT parts exist when content systems only generate HTML, and adding tracking pixels or wrapping links with click trackers.
The functionality works bidirectionally – useful for anyone running inbound mail infrastructure or hosting mailboxes.
The trade-off: Tom Mairs notes their recommendation remains to scan and modify messages before they hit the MTA, but acknowledges corporate policies sometimes require last-mile verification. The inline processing will consume CPU cycles, so capacity planning matters.
Technical details: The parse_mime function returns a table of MIME parts that can be manipulated using KumoMTA’s regex and string handling routines added in earlier releases.
Full technical documentation: Click here for more details
