When Virtualmin adds per-IP SSL configuration for Postfix, it's currently adding smtp (25) and submission (587) to master.cf.
In addition to these, port 465 is (was) commonly used before STARTTLS came about, or at least became widely supported, and is apparently still in use at some mail providers for compatibility reasons. Wikipedia specifically mentions Entourage 10 and Outlook for Mac 2011 (https://en.wikipedia.org/wiki/SMTPS). Postfix does support this type of configuration with "-o smtpd_tls_wrappermode=yes" (in addition to the other configuration set for smtp and submission).
Glancing at server/copycert-lib.pl I do actually see mention of this wrappermode configuration in Postfix, but if I'm reading it right this appears to be more of a global thing. virtual-server/feature-ssl.pl only deals with smtp and submission for Postfix, and no configuration other than this is added to my master.cf when I add domains to our servers.
As is mentioned on Wikipedia, 465/SSL has been deprecated for a while now, so I'm not really sure what your thoughts would be on including this in Virtualmin. I've personally found this configuration to be useful on our internal corporate mail server, and I have been able to add this to our Virtualmin test servers with some post-add/modify/delete hooks, but it could be useful for others as well so I thought I'd post a feature request here.