[Dovecot] resilience suggestion

[David Lee]

On the whole we are pleased with our trials of dovecot to replace UW-IMAP.

But (ah!) we have hit one particular problem, in which we think dovecot
could probably benefit from a resilience improvement.

We're running dovecot on Fedora Core 5 (FC5), with passwd map details
supplied by NIS.  We have found that "nscd" sometimes thinks that a
username is invalid, even though it is valid.  So when "deliver" attempts
a delivery to the INBOX of that username, it receives "user unknown" from
the name service, and then does a 5xx permanent failure of valid email.