RFC2822 requires that each email message have a unique worldwide identifier associated with it. That identifier is listed with the Message-ID: header (Message-ID:) and often looks something like this:

Message-Id: <200212131533.dAPFX1o01876@here.us.edu>

The information inside, and including, the angle brackets is the message identifier. That identifier is what is listed with the msgid= syslog equate:

msgid=<200212131533.dAPFX1o01876@here.us.edu>

If a mail message arrives without a Message-ID: header, and if your configuration file correctly includes a definition for that header, a new identifier will be created and listed with msgid=. If a Message-ID: header is absent, and if your configuration file incorrectly excludes a definition for that header, the msgid= syslog equate will be excluded from the syslog report.

The msgid= syslog equate is shown only for sender records.