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

Arnt Gulbrandsen wrote:
> 
> 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.

It does not know that final delivery will happen.

An "MX" host is an incoming boundary MTA (see email-arch) for email.  It 
typically does not know all of the valid addresses within the organization and 
it certainly cannot know about all of the possible errors that could prevent 
delivery, between the time the MX host takes receipt and final delivery finally 
happens (or doesn't.)


> If you've delivered to the best MX, then that's as far as SMTP takes the 
> message. 

This, too, is often incorrect.  Between the boundary MTA and final delivery can 
be multiple email hops, all using, or not using, SMTP.


> 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.

Folks seem to think that receipt by a host listed with an MX in the (presumably 
public) DNS is of fundamental import to for the end-to-end process of posting 
and delivering mail.  It doesn't.  It is merely one of a potentially large 
number of hops along the way.  In fact, the email architecture has little or no 
technical details that pertain to the operation or semantics of a boundary MTA...

d/

-- 

   Dave Crocker
   Brandenburg InternetWorking
   bbiw.net