MID module

The purpose of the MID module is to suppress the INVALID_MSGID (malformed Message-ID header) and MISSING_MID (missing Message-ID) rules for messages which are DKIM-signed by some particular domains. The module is disabled by default and requires a source map to be configured before it becomes active.

Configuration

The module requires a source parameter pointing to a map file. Without it the module logs a warning and disables itself at startup.

Configuration options

Option	Default	Description
source	required	Map of domains and optional Message-ID patterns
symbol_known_mid	KNOWN_MID	Symbol for known Message-ID pattern match
symbol_known_no_mid	KNOWN_NO_MID	Symbol for known missing Message-ID
symbol_invalid_msgid	INVALID_MSGID	Symbol to suppress for invalid Message-ID
symbol_missing_mid	MISSING_MID	Symbol to suppress for missing Message-ID
symbol_dkim_allow	R_DKIM_ALLOW	DKIM allow symbol to check
csymbol_invalid_msgid_allowed	INVALID_MSGID_ALLOWED	Composite symbol for allowed invalid Message-ID
csymbol_missing_mid_allowed	MISSING_MID_ALLOWED	Composite symbol for allowed missing Message-ID

Example configuration

mid = {
  source = "${CONFDIR}/mid.inc";
}

The source setting points to a map to check DKIM signatures (& optionally message-ids) against, formatted as follows:

example.com /^[a-f0-9]{8}(?:-[a-f0-9]{4}){3}-[a-f0-9]{12}-0$/
example.net

With this configuration, when a message is DKIM-signed by example.net (no Message-ID pattern required), KNOWN_NO_MID is inserted. When signed by example.com and the Message-ID matches the regex, KNOWN_MID is inserted instead.

The module then registers two composite symbols that trigger alongside the respective base symbols:

• INVALID_MSGID_ALLOWED fires when both KNOWN_MID and INVALID_MSGID are present.
• MISSING_MID_ALLOWED fires when both KNOWN_NO_MID and MISSING_MID are present.

By assigning a sufficiently negative score to these composite symbols in your scores configuration, the positive score of INVALID_MSGID or MISSING_MID is counteracted. The base symbols themselves are not removed — the net score is reduced to a neutral level via the composite.