I have dovecot imap server and exim4 running on Debian Buster. I am using exim4 for smtp, but at the Debian upgrade before last (early Jan 2021), Debian arbitrarily turned on submission in dovecot by adding the file /usr/share/dovecot/protocols.d/submissiond.protocol
. That blocks exim from properly servicing smtp.
It took me ages to find this was the cause, but once I had, I renamed that file to submissiond.protocol.unused
and all was well. However, I did a Debian upgrade again today (with apt) in response to a security alert in an unrelated package, and it put that file back again and broke smtp once more.
Clearly I am not turning this protocol off in the correct way. What should I do to turn it off in such a way it stays off over Debian upgrades?
If you don't need the Dovecot submission daemon, you can just remove the package that provided that functionality.
If apt didn't restart dovecot during the removal, you will need to do so yourself.