Dave CROCKER wrote:
POP has been abused for last-hop delivery to intermittently
In fact, during the development of the Internet Mail Architecture
document, I argued rather vigorously that it really /is/ a last-hop
delivery protocol, rather than being and 'access' protocol that parallels IMAP.
Dave, I don't particularly recall this debate, but it sounds this
would of been a +1 from me.
Community consensus went vigorously against me on that. Certainly typical
implementations don't really support a 'last-hop relay' model well or at
all, as evidenced by DSN being issued /before/ POP pickup...
I suggest it might depend on much old school material in retaining
UUCP queuing, holding and relaying logic in the implementations.
There is no DSN when mail is accepted and stored for a locally hosted
user - a behavior mode of the receiver MTA was as a MDA (last hop).
This user-bin mail will normally only be accessed by MUAs. If not, it
may expire per local policy mail storage policies such as:
Days to keep Received mail: 21
Days to keep Unread mail: 14
Of course, these days, those numbers are higher or even disabled (0).
If you are locally hosting other domains, then ala UUCP, it would be
stored for the domain to pickup or your mailer schedules a UUCICO event.
When SMTP was added, the choice became per hosted domain:
(o) Route via SMTP
(_) Route via UUCP
I don't see MUAs here (last hop final destination) hence DSN is a