Re: [PLUG] user's mail not being delivered, odd error

[Rich Kulawiec <rsk@gsp.org>]

This is speculative but is based on way too many years of running mail
systems:

Temporary errors like this which result in deferrals (not rejects) often
have a root cause in DNS lookup failures.  The code in most sensible MTAs,
like postfix and sendmail and so on, is written to expect these and to
do reasonable things -- like queue and retry later.  This behavior will
persist across a reboot (which matches your report) and will persist
until messages are deemed to have been queued long enough that giving
up and returning a permanent failure is indicated.  ("long enough" may
be hours or days.)

So this is where I suggest rule #1 of debugging mail systems: check
DNS resolution forwards, backwards, and sideways.  If it's functioning
correctly, then you won't have solved this problem, but you will have
exonerated the most frequent root cause and can move on to other
candidates.  This would also be a good time to see if anybody made
Just One Minor Unimportant Change That Won't Affect Anything to any
DNS records or resolver configuration right before the problem surfaced.

---rsk
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