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message:check_fix_conformance(CHECKS, FIXES)

Since: Version 2023.11.28-b5252a41

The functionality described in this section requires version 2023.11.28-b5252a41 of KumoMTA, or a more recent version.


Fixing messages with this method is inherently imperfect: it is based on a deliberately relaxed interpretation of the message content and it is possible, or even likely, that non-conforming input is parsed in a way that results in omitting certain details from the original input.

The purpose of this method is as a best-effort convenience for correcting minor and obviously recognizable issues that cannot easily be resolved at the message generation stage.

It is recommended that you carefully evaluate the effects of this method before deploying it in production.

This method serves two related functions:

  • To check RFC conformance issues for which you wish to reject the message.
  • To correct RFC conformance issues in case you wish to accept the message.

It accepts two parameters that encode the set of conformance issues that are applicable. The conformance set is represented as a string listing the issues separated by a | character:

kumo.on('smtp_server_message_received', function(msg)
  local failed = msg:check_fix_conformance(
    -- check for and reject messages with these issues:
    -- fix messages with these issues:
  if failed then
    kumo.reject(552, string.format('5.6.0 %s', failed))

The set of supported conformance issues is:

Label Meaning
MISSING_COLON_VALUE A header was listed with only its name, and without any value. eg: "Subject" instead of "Subject: the subject"
NON_CANONICAL_LINE_ENDINGS The message contained line endings that were not in the canonical CRLF form required by SMTP
NAME_ENDS_WITH_SPACE A header name ended with space instead of a colon. eg: "Subject :" instead of "Subject: ". While that is valid for HTTP, it is invalid for email.
LINE_TOO_LONG The line length for the body portion exceeds the MIME message wrapping width that is intended to keep message text wrapping within 80 columns.
NEEDS_TRANSFER_ENCODING The parsed content includes 8-bit content and thus needs to have transfer encoding applied in order to safely transit the 7-bit SMTP network
MISSING_DATE_HEADER The "Date:" header is not present
MISSING_MESSAGE_ID_HEADER The "Message-ID:" header is not present
MISSING_MIME_VERSION The "Mime-Version:" header is either not present or is set to some other value than "1.0"

The way this method works is that it will attempt to parse the data associated with the message into a MIME tree. The parsing stage will accumulate the set of conformance issues it uncovers as it parses the tree.

Then the CHECKS parameter is decoded into the set of issues for which an error needs to be generated. If there is an intersection between CHECKS and the discovered conformance issues, then an error message is generated, listing the problematic issues. Here's an example:

Message has conformance issues: LINE_TOO_LONG

That message is returned from the method as a string so that you can then choose to issue an appropriate error response. For example:

local failed = msg:check_fix_conformance(checks, fixes)
if failed then
  -- Rejects with an error like:
  -- `552 5.6.0 Message has conformance issues: LINE_TOO_LONG`
  kumo.reject(552, string.format('5.6.0 %s', failed))

If after processing the CHECKS parameter an error was not returned, the FIXES parameter is consulted; if there is an intersection between FIXES and the discovered conformance issues, then the message will be "fixed".

The strategy for fixing is simple:

  • If any of the problems are not due to missing headers:
    • The message will be rebuilt from the parsed tree. This will cause headers and parts to be re-encoded to follow the best practices coded into the built-in mime message builder.
  • Missing headers will be synthesized and added to the rebuilt message
  • The resulting message will then be re-encoded as a byte stream and assigned as the data that will be saved when the message is ready to be spooled.


Since fixing issues other than missing headers essentially rewrites the message, the chances are very high that any digital signature in the original message will be invalidated.