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RE: Recipient is offline

2011-09-01 20:16:47

--On Thursday, September 01, 2011 16:14 -0700 "Murray S.
Kucherawy" <msk(_at_)cloudmark(_dot_)com> wrote:

Because this would mean that any user who turns his SMTP
server on for a couple of hours each day would receive his

As long as there's a retry during that window, yes.  But that
isn't guaranteed.

And, if the sender is doing with 5321 (and 2821 and 1123)
suggest (I'm not going to have a discussion about what is and is
not normative in 5321, at least today), the odds of a
rarely-available receiver getting delivery go down rapidly with
time.  Even if the sender keeps trying for more than the
recommended 4-5 days, incremental back off strategies may make a
short receiver-online connection period an increasingly elusive

See the discussion in RFC 5321, Section, for additional
hints about this or, as others have suggested, seriously
consider an intermediate server that is usually or always on-net
and a mechanism for working with it when your
intermittently-connected host is online.  Those could be as
simple as having the intermediate server know the typical
schedule for the intermittent host being online as well as
mechanisms like ETRN or pinging the intermediate with some sort
of message.


I'd say that most probably try for 3 days (gives admins
chance to fix their server after a weekend if it broke on the
Friday evening). Busy ones like gmail could try less because
they have so much mail, and don't care as it's a free
service. As long as they generate an NDR it's compliant with
the standard.

After some little-known sysadmin trickery I learned a long
time ago[*], I managed to figure out that they do indeed try
for at least a couple of days, though the frequency of retries
is not constant.  They certainly don't give up after only a
few seconds.

However, if a mail server is online for only a few hours a
day, then I'd seriously suggest getting a backup mail server
to accept messages during the downtime (or putting the main
mail server somewhere else with a better connection), unless
you don't really care about getting mail...

I concur, especially if it's one that lets you do ETRN
(RFC1985) or some other demand-based delivery system.


[*] I asked them.

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