Noted. I will check this later. Thanks for the feedback.
With dovecot 2.3 Diffie-Hellman is necessary. Otherwise email clients may display this error in the logs:
Otherwise email clients may display this error in the logs:
Jul 9 11:27:52 mail dovecot: imap-login: Error: Failed to initialize SSL server context: Can't load DH parameters: error:1408518A:SSL routines:ssl3_ctx_ctrl:dh key too small: user=<>, rip=2804:xxxx:xxxx:xxxx:e4c8:f463:d763:1130, lip=2804:xxxx:xxxx:xxxx::163, session=<kv2pXUCNlcsoBBBUDxgAAOTI9GPXYxEw>
For create dh.pem I used: openssl dhparam -out /etc/dovecot/dh.pem 2048
After I created and configured the dh.pem, I had to recreate my imap accounts in thunderbird to normalize the accesses.
I updated my Debian Stretch to Buster and only noticed problems with dovecot, as I explained above. The rest seems to be working. fail2ban working!