You can do that via a digital ID from verisign - costs about $9 and plugs into outlook, if they have a digital id then you can install keys on each others machines and send email in true 128bit encryption.

We had one on all the support email addresses at one stage (dunno if anyone ever renewed them) - a draw back to digital id's were that they only worked in outlook (possibly eudora and netscape), but in some cases caused the other users machine to totally freeze up to 5 minutes before then displaying the email (this was back in 2000 btw- so they could have worked that bug out).

Not sure about a website - would be pretty complex.