Opendkim won't start: can't open PID file?

I fixed it by specifying the PidFile in the /etc/opendkim.conf file:

PidFile                 /var/run/opendkim/opendkim.pid

I think it's better to solve it here than modifying the systemd script. Also, systemd recommends the PIDFile option if the Type option is set to forking, which it seems to be in this service.

This PidFile setting was present in the default /etc/opendkim.conf file. However, I replaced that file while I was following a tutorial for configuring OpenDKIM. So that is where it went wrong for me.

Since this daemon runs alone, systemd doesn't need anything special to manage it. Importantly, having a "PIDFile" directive suggests that it's a more complicated service, and will put a PID into the PIDFile on it's own.

The solution is to remove the PIDFile line. Now systemd handles it just fine.