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:- This editing should consist of adding the last line given here.
- Restart your mail service in 'Server Admin,' or restart your server, and you should be golden.
- You will have to edit the mail clients' SMTP preferences to use this port. If your client is Mail.app, this is simple and in the 'Accounts' pane of 'Preferences.'

