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.
From an operational standpoint, a single-hop design makes many other
parts of the design much cleaner. It is also easier to explain to
users.
Of course, if the receiving MTA wants to re-send a message to another
MTA instead of writing it to the message store, that's fine. However,
doing so is not part of the specification. That is, the protocol
specifies just originator->origMTA->recMTA->receiver or
originator->recMTA->receiver, but does not specify
origMTA->otherMTA->recMTA.
--Paul Hoffman, Director
--Internet Mail Consortium