Security with Exchange Server 5.5

I am running Exchange Server 5.5 and am trying to figure out the best way for users to encrypt email. I checked out Verisign and that will work for the exchange client with a lot of administrative overhead initially. Is there an easy way to do something similar with exchange server? Also, with security, in order to encrypt a message, I must already have the recipients digital ID? I’m sure most of the people we email do not have this. Isn’t there a way to encrypt a message w/o have the recipients ID?