The mail server admin panels in Panther Server have changed from their equivalents in Jaguar Server, hiding the options to modify the port on which Postfix listens for SMTP connections. Combined with the trend of more ISPs blocking traffic on port 25, this has become a more popular problem.
The solution:
As root, edit the file /private/etc/postfix/master.cf, so it looks something like the following. I use port 2525 in this example; you can substitute it with any port you'd like to use...
# =====================================================================
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (yes) (never) (100)
# =====================================================================
smtp inet n - n - - smtpd
2525 inet n - n - - smtpd
NOTES:Mac OS X Hints
http://hints.macworld.com/article.php?story=2004122809382235