Hello Peer,
TLS is part of the normal mailflow, and therefore does not need a specific feature key. The encryption feature you are talking about is the secure envelope that our Cisco Registered Envelope Service (CRES) is providing, where the message itself is encrypted before going out to the recipient. But that's not related to TLS used for securing a connection, with a normal functioning IronPort appliance, you can use TLS for sending and reciving, you can also configure TLS and SSL versions and ciphers if nessesary.
Hope that helps,
Andreas
