what about port 25 blocking that is now done by many access providers?
this makes it impossible for mobile users, coming from those providers,
to access your server and do the auth.
amb@cinephile:~$ fgrep submission /etc/services
submission 587/tcp # submission
amb@cinephile:~$ fgrep ssmtp /etc/services
ssmtp 465/tcp smtps # SMTP over SSL
Alex