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user_id=6405

Ah, righto. Yes, the message-id is supposed to be 
<some.string@SHOST> where SHOST is the sender's host. It's 
actually supposed to match the host part of the Sender: 
header, but that's obviously either changed in a later RFC 
or is just ignored (a few sample e-mails I just looked at 
had all sorts of combinations of things.)

Upshot: I'll change it so the format is just 
"<%s.%s.%s%s@%s>" with the current arguments. Note the 
addition of the angle-brackets too...