|3.1. Submission Identification
Port 587 is reserved for email message submission as specified in
this document. Messages received on this port are defined to be
submissions. The protocol used is ESMTP [SMTP-MTA, ESMTP], with
additional restrictions as specified here.
While most email clients and servers can be configured to use port
587 instead of 25, there are cases where this is not possible or
convenient. A site MAY choose to use port 25 for message submission,
by designating some hosts to be MSAs and others to be MTAs.
This information comes from http://www.ietf.org/rfc/rfc2476.txt