I'd say it was safer to completely disable sending... otherwise a spammer could write a script to send a message one-by-one instead of to many recipients at once. A simple page or notice saying "For security reason, sending has been disabled in the demo" should be sufficient.
Why not just allow sending mails to the demo account? This way everybody can easily see how sending and receiving mails works (as in: looks like..) without endangering the rest of the internet :]
Cheers, Niels