Operations requirement: no hop-by-hop

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