Port 465/tpc is noted as SMTP over TLS, and is quite an appropriate port
to tell sendmail to listen on as well, though TLS should be enabled. :)
I'm a big fan of SMTP/TLS and IMAP/SSL on server side, so I can just
tell my clients to use encryption always, no mess with ssh, and the port
465 one wasn't blocked by my ISP that blocked port 25.
If you can't do TLS/SSL it's no matter. Have mail client listen on any
port you like. I've had postfix listen on 25, 465, and 25025 all at
You probably want to find a way to make the one sendmail process run on
more ports, rather than running multiple instances of sendmail.