Dave Crocker writes:
The email-arch specification notes that Internet standards provide for at
most one Message-ID, but the document asserts that more than one sometimes
appear in a message. I put that in because that's been my impression.
Sometimes they do appear. It is forbidden.
The less discussion about what-if-and-when, the better.
I see one such message with duplicate Message-ID per 200.000 messages
on the average at our site (last two months average). In almost every
case a messages originates from MUA:
QUALCOMM Windows Eudora Version 7
and rarely from:
Microsoft Office Outlook 11 - or 12.0
At least with Eudora there is apparently a bug which produces
a duplicate (and different) Message-ID on mail forwarding.
So I'm going to remove reference to them from the email-arch draft.