Forgive me but I only partially agree
Ok for the sending operations, and I had already done it.
But also for client operations
In fact, as by the way you specify yourself here
https://portal.smartertools.com/kb/a2671/configure-ssl-tls-to-secure-smartermail.aspxset of port mappings for 25 (TLS), 110 (TLS), 143 (TLS), 465 (SSL), 993 (SSL), 995 (SSL)
Then setting the standard ports 25 110 143 with the tls (which does not become mandatory but supported)
in fact also on your mailserver
openssl s_client -starttls smtp -crlf -connect mail.smartertools.com:25
openssl s_client -starttls pop3 -crlf -connect mail.smartertools.com:110
openssl s_client -starttls imap -crlf -connect mail.smartertools.com:143
notice how it accepts the tls protocol
From the documentation found on your site and on the community too often it seems that it is not recommended to configure ports 25 110 143 with tls (but maybe I have misunderstood me)