So I would suggest offering SMTP (AUTH) support on ports 25 and 26, just to be sure.
No no no.
RFC: 2476:
| 3. Message Submission | 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.
Port 587 has been widely deployed:
$ telnet smtpauth.bluewin.ch 587 $ telnet mail.gmx.net 587 $ telnet smtp.gmail.com 587
Inventing new ports < 1024 is just plain wrong.