Mailserver and Nameserver change
End of 2012 my provider netcup decided to change
their nameservers. In an email to all their customers they explained that
/etc/resolv.conf should be changed accordingly.
Unfortunately they didn't state that already running daemons (like postfix - my MTA) would not benefit from that change. So it was only by accident that I discovered, that outbound mails wouldn't be sent any more. The reason was that postfix was unable to find the MX records for domains he should be sending mails to.
It was an easy fix - just restarted postfix (and dovecot, just to be sure) and I learned some nifty commands while I was at it.
mailq- lists all mails queued currentlypostfix flush- tries to send all deferred mails now
Note: debian systems have scripts under /etc/resolvconf that should be
executed whenever resolv.conf changes. In my case it would feed the new
/etc/resolv.conf to postfix and do a postfix reload.