How do I config TLS/SSL on Exchange 5.5?

We have a client that requires TLS encription on email between out companies. We currently use exchange 5.5. How do I configure TLS/SSL on messages between these two companies only without affecting other clients/messages? We currently use Postini for spam filtering and they offer TLS however the client does not want to use that. They have reqested that we create subdomain accounts called tls.mydomain.com to use for this.