Go to your DNS Server: Create A record "mail.mysite.tld." and pointing it to IP of your mail server. So basically, you can create as many smtp/imap/pop server name as you like (you can create as many sub domain or even other domain and pointing to the IP as well).
But if you want to use commercial SSL for only one subdomain/domain then you must use that subdomain/domain as your SMTP/IMAP/POP server name.