This just looks like a broken sender - I mean, RFC5322 is pretty clear that "The generator of the message identifier MUST guarantee that the msg-id is unique."

This just looks like a broken sender - I mean, RFC5322 is pretty clear that "The generator of the message identifier MUST guarantee that the msg-id is unique." I know that practically, we want to support every email out there, even if it is not quote RFC compliant -- but how far do we want to go? Is it worth adding all new mechanism just because someone out there may be sending non-compliant messages? Especially given that those non-compliant messages are likely to break other things like mail databases, IMAP UIDVALIDITY and so on?