Encrypting mail with GPG and using the built in encryption of SR is not the same thing. 99% chance SR is just running things out of an encrypted mounted container. Mounted is the key word here. It is pretty much worthless to use encryption in this way without also having tamper resistant systems protecting the servers memory as well as physical intrusion detection systems. Even if SR also has these systems in place, it still isn't as good considering if the server is rooted an attacker can get the messages in plaintext, unlike with GPG encrypted messages where they would need to root the clients they are interested in.