BTW: Received header fields have always been discussed (not always in a
consistent way) in both the x821 and x822 documents (and probably
earlier than that as well). So you do need to look in both documents to
be sure what you propose is OK :)
Yes, I looked. The syntax in RFC 5322 is considerably looser than in
5321, so 5321 is the spec that matters in this case.
I'm not unalterably opposed to [ip]:port but I worry that it would break
more header parsers than adding a port or lport/rport clause.