Even with TLS, that is usually handled by issuing STARTTLS on TCP port 25. I can't find anything in the RFCs mentioning server-to-server smtp delivery happening on anything but port 25? Do you have a reference for that? In fact, even the MX for google domains (aspmx.l.google.com) does not listen on TCP port 465 or 587, only 25.
I'm not certain, so you might be more informed than me. It's possible my server is sending on port 25 and since the firewall I use doesn't block outgoing connection I just didn't notice. TIL!
