SpamAssassin Rule: MISSING_MID
Standard description: Missing Message-Id: header
The message does not contain a Message-Id header. The relevant standards specify that mails should have Message-Id headers. All properly written Mail User Agent (or Mail Submission Agent) software is expected to add a Message-Id header.
This suggests that the mail was sent by badly-configured mailout software rather than by a normal email client.
The default scores for this rule can be found in the online list of tests.
The Message-Id header is detailed in RFC 2822 sec 3.6.4.