Excerpts from mail: 26-Mar-92 Re: MIME types for ack[-req..
Keith Moore(_at_)cs(_dot_)utk(_dot_)edu (2652)
Depends on what they are being used for. Transport-level acks
are used to
check the health of the message transport. If I maintain a mailing list,
all I care about is that my messages get to the recipient's mailbox. If
the
recipient never reads them it's no concern of mine.
The problem is that knowing what is "the recipient's mailbox" is pretty
much impossible in the general case. For example, when UNIX sendmail writes
something into /usr/spool/mail/username, it thinks of that as the recipient's
mailbox, and the case is closed. However, I know of cases in which a gateway
to another mail world is implemented as something that checks /usr/spool/mail
files and picks things up from there, in which view /usr/spool/mail is just an
intermediate queueing area. In general, MTA's simply don't know whether their
"delivery" of mail is final or intermediate, which is why I think this kind of
ack isn't useful even in the case you describe. You can even get a positive
"it was absolutely delivered" ack from sendmail in such a case, and then get
back a REJECTION from something further downstream.