I’ve got Postfix setup on my local desktop computer and on my server. On my local desktop, every now and then Postfix stops sending e-mails. The /var/log/syslog gets filled up with DNS related errors:

Nov 22 20:24:49 box postfix/smtp[5180]: 09C212FC0859:
to=<email-address>, relay=none, delay=69887,
delays=69887/0.01/0/0, dsn=4.4.3,
status=deferred (Host or domain name not found.
Name service error for name=ngeor.net type=MX:
Host not found, try again)

I know there’s nothing wrong with my server, because it can pick up e-mails correctly from other sources, so it must be something wrong with Postfix on my local desktop.

Digging around in the usual places (/etc/resolv.conf) didn’t help much, after all I didn’t experience any other DNS problems using my desktop.

So, TIL: Postfix is actually using its own resolv.conf, which can be found in /var/spool/postfix/etc/resolv.conf. And in my case, that file was empty…

I’m not sure what caused the file to become empty, so I’ll keep an eye for that.

I did find a script in /etc/resolvconf/update-libc.d/postfix which copies the /etc/resolv.conf over to Postfix’s resolv.conf, so I’m assuming that something went wrong there.

Additionally, when running Ubuntu as a desktop, it typically uses its own local caching DNS server, which isn’t the case with the server version. So this could also be causing some problems.

For now I just copied over the correct contents from /etc/resolv.conf to Postfix’s resolv.conf, which solves the problem (until next time that file somehow gets empty).