I noticed a significant increase in the number of hosts attempting to authenticate to my Exim submission ports (465 and 587). As a result, I have implemented a number of security measures. These measures have effectively blocked the attempts. This post outlines the main measures that I have implemented. This includes ACLs and fail2ban blacklisting.
Configuring a common login for Dovecot IMAP(s) and Exim SMTP. Also Exim Submission port and TLS (SSL) configuration.