It seems simple enough to block all outgoing SMTP to anything but your own server, and to apply such a filter on your dialups. I don't know if the more complex solution you propose is worth it -- the users will quickly learn where they should aim their mail...
Experience suggests that's not true -- users whine very loudly that their mail used to work and it doesn't work any more. Telling them that the config that they copied from a friend down the hall, which bounces the mail off the server of some company where the friend used to work, was bogus in the first place rarely impresses them. That's the point in making it transparent. Also, it's for the benefit of people who use these global roaming setups, so they don't have to reconfigure Eudora every time they get off an airplane. Regards, John Levine, johnl@iecc.com, Primary Perpetrator of "The Internet for Dummies", Information Superhighwayman wanna-be, http://iecc.com/johnl, Sewer Commissioner Finger for PGP key, f'print = 3A 5B D0 3F D9 A0 6A A4 2D AC 1E 9E A6 36 A3 47