Can anybody think of additional considerations (other than
the transparency issue: what happens, if one/some/all of the MUAs, MTAs
involved in generating, transporting, delivering or digesting the
message
do *not* understand this header field?
agreed. in general, only the sending and recipient MUA should have to
understand the new field; MTAs shouldn't have to care about it. if
you're defining new functionality for MTAs it generally doesn't belong
in the message header. but in reality there are some exceptional
cases. for instance MTAs do rewrite addresses (even though they
shouldn't) so this has implications for any new field that has
addresses. and with the to-noreply and cc-noreply proposal it became
obvious that the submission server would also have to understand them
(at least for sendmail -t scenarios).
so basically any new proposal for header fields should analyze its
impact on sending MUA, submission servers, intermediate MTAs, final
delivery agents, and recipient MUA (including how this affects replies
if applicable). when applicable, it should also analyze the impact on
mailing lists, forwarding, mail filters, and other intermediaries, and
auto responders.