I have recived some messages with an empty message-id. Those messages
were all junk email, so can I use the empty message-id as a filter for junk
email?
Usually they'll match, so yes. There was discussion about this on
one of the mailing lists -- procmail or spamtools. Anyhow, the
rule that I use to check the Message-Id header is:
#Incorrect or failed Message-Ids:
:0:
* !^Message-Id:.*<[^<>@]+(_at_)[^<>]+>
/scratch/spam
This essentially checks for RFC 822 compliance. I've had 95% success
with this -- I received one valid message that had a totally weird header,
so it was counted as spam.
Chris