Re: mail relay was: Re: Intent to revive "expires" header from draft-ietf-mailext-new-fields-15

Jeff Macdonald writes:
> Ok, you both lost me. Or Perhaps you are agreeing with me. How does a 
> sending MTA know that final delivery has happened?

It only knows that final delivery will happen, not that it has happened.

If you've delivered to the best MX, then that's as far as SMTP takes the 
message. The message may have a long way to go, but that's independent 
of SMTP and DELIVERBY. For example, the recipient may use something 
like Thunderbird offline, in which case the recipient's inbox at the 
server is just another spool, emptied when the recipient polls.

The recipient's SMTP server won't know that, won't be able to control or 
prevent it. Since that's true even if it advertises DELIVERBY, I think 
it's acceptable to deliver to the best MX in R mode.

Arnt