Almost all communication between mail servers on the Internet is done on port 25. This the well known port.
See rfc4409, it reserves tcp/587 as an alternate submission port for SMTP. Port 25 and 587 are reserved for SMTP, and Submission through IANA http://www.iana.org/assignments/port-numbers.
SMTP can be secured via TLS over port 25, but this doesn't seem to be widely used for Internet email, but I know it is used for some internal communication on some networks.
Many ISPs seem to also accept submission on tcp/465, but this port isn't officially registered for this purpose.
Of course, any mail server could listen for SMTP on any port, but a sending server won't have any method to discover the alternate port and must assume that it is on port 25.
I know some organizations accept submissions from mail clients on other unusual ports like 2525 and others.
