This may or may not be controversial, but I propose that a
requirement for sane operations is that mail-ng be single hop.
Multi-hop was a good design 20 years ago when many entities were not
online all the time, but it has also proven to be a huge hurdle in
extending SMTP.
it also hurts reliability and makes it difficult to get some kinds of
service that we'd like to have - such as the ability to get immediate
confirmation of delivery and the ability to negotiate content between
sender and receiver (without having to trust an intermediary to do
the negotiation).
though I doubt that single hop is quite achievable. I do think that
sndr MUA -> submission MTA -> rcpt MTA/message-store -> rcpt MUA
is fairly reasonable.
I don't see that this is a requirement, but I do think it would be
a useful design choice if we could make it work.
--
He not busy being born, is busy dying. - Bob Dylan