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 default configuration of this module is shown below:
mid = {
url = [
"${CONFDIR}/mid.inc",
];
}
The url setting points to a list of maps to check DKIM signatures (& optionally message-ids) against, formatted as follows:
example.com /^[a-f0-9]{8}(?:-[a-f0-9]{4}){3}-[a-f0-9]{12}-0$/
example.net
With this configuration scoring for INVALID_MSGID and MISSING_MID symbols is removed if the domain is DKIM-signed example.net or the domain is signed example.com and the message-id matches the specified regex.