Dave Crocker writes:
So, Community, I'm querrying you:
Do you see multiple message-id's in messages?
Yes, but very seldom. On the order of once per million messages.
If you do, what affect do they have?
There are two varieties.
Sometimes I've seen repeated message-id fields. The exact same ID
specified twice in the header of the same message. No effect.
Messages with two different message-ids also occur. In the real examples
I've seen, both IDs were usually added in the same administrative
domain, although sometimes I wasn't sure of that. This has little or no
effect. More MUAs construct References from the first ID than from the
I've never seen a message with more than two Message-ID fields in the
All this based on my corpus and YMMV and have a pleasant Sunday.