I see posts on the internet, Google Apps Engine, and a few others that make mention of this unusual client submit port.
Can anyone clarify which is the oddball port I should support (in addition to port 587) for end user email submits?
Port 456 has nothing to do with e-mail.
Port 465 is SMTP over SSL. But that is for MTA to MTA communication only. Clients should never use it... Unfortunately some MSAs are programmed to use it. MTAs basically never use it unless configured specifically to do so. SMTP with TLS is fairly commonly supported now and somewhat widely used.
Port 587 is the one any only port MSAs should be submitting new mail on. TLS should be supported and used if the clients want some sort of security (mainly for securing their password as the MSA port should not accept e-mail from anonymous connections).
Port 465 is a legacy port for S/SMTP. I've seldom seen it used to begin with and not actively for years now. I would consider it safe to disable this port completely.