At 10:08 -0400 on 09/02/2011, John C Klensin wrote about Re:
Recipient is offline:
And I was told to put the text into 5321 that pointed out that using
an inbound
email message to trigger a delivery attempt for messages in the
queue might avoid the need for ETRN (or TURN in some cases). I
didn't make that up although I've seen it put to good use.
For this to work, there would need to be a list maintained by the
delivering MTA that lists which other MTAs it has tried to deliver
to which failed due to failure to connect (along with a queue of
messages that need to be delivered to that MTA and a time to next
attempt to connect). When a new message arrives and it gets delivered
(since that connect attempt worked) then the queue of pending
messages can then be immediately flushed to the MTA. How many MTAs
have this type of support to force flush a pending queue when a
message delivery succeeds? I know that ETRN support will do this but
that is due to the intended recipient MTA having connected to the
delivery MTA and saying to it "I'm here to receive the pending
messages - Send them now".