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?