Encrypting Mails

How do i send encryt mails to other domains, I am using exchange 2000 with outlook 2003, I need to configure soem users ID to send mails to internet with encryption and digitally sign. For Example say User1@trail.com,User2@taril.com,user3@trail.com need to send encrypted mails with digital sign to Auser@in.com,Buser@in.com.
and if its digitally sign how to the person reciving will be able to open,