postfix: milter-reject: END-OF-MESSAGE from ...: 4.7.1 Service unavailable (opendkim)

The problem, in my case, was that the signing key file had root owner, so doing:

sudo chown opendkim:mail /etc/opendkim/selector.private

Fixed the problem.

If this is not your problem, don't forget to take a look at the journal, for both! postfix and opendkim (i forgot to lookup opendkim and there it was the error):

journalctl --follow --unit postfix.service --unit opendkim.service