After having done a dist-upgrade fo Jessie to Stretch OpenDKIM didn’t start any more.
After research I found the answer which worked for me in this site:
I’m using the ‘inet’ socket for the communication between Postfix and OpenDKIM at port 12345.
eg. My config in of OpenDKIM in Postfix:
milter_default_action = accept
milter_protocol = 6
smtpd_milters = inet:localhost:12345
non_smtpd_milters = inet:localhost:12345
Solution found in the above web site:
service opendkim restart
Note: There are other solutions for the ones that use other kind of communication sockets for the communication between Postfix and OpenDKIM found in the same above site.
Solution: Regenerate the proper