What is the client wanting to encrypt?
if it's the email stored on the server then at the moment this isn't built into SM see the thread here https://portal.smartertools.com/community/a592/encryption-of-mail-boxes.aspx
if it's the actual email itself (requiring decryption by the receiver) then again this isn't built into SM but have a look at http://lifehacker.com/how-to-encrypt-your-email-and-keep-your-conversations-p-1133495744 to get started on how to do this. You also should check to ensure the server has been updated to disable the schemes that are no longer secure. See the Qualys.com site for their ssl checker.
hope this is some help
if it's the email whilst in transport I.e the connection between the sending and receiving server. Then this is a server wide setting and requires a valid certificate (which doesn't have to match the individual domain). This is becoming (if not already) the norm. However, it does require both ends to support a common encryption scheme. Also you should check that