There are a few steps on implementing the on-box encryption and they are well-documented in the AsyncOS user guide.
1. Log into the Support Portal
http://www.ironport.com/support/login.html
2. After you've logged in, on the left side under "Appliance Documentation", click on "Email".
3. Download the AsyncOS regular user guide and search for "IronPort Email Encryption"
4. All the details you need and steps on how to implement it will be there.
How can I configure Email Encryption on the C150 like the PXE Encryption
