Currently Symantec Encryption Desktop does not support encryption via IPv6. If you specify to send via IPv6 the email message will send in the clear as we currently do not have any proxy in place to detect the message.
For request for IPv6 support on the Symantec Encryption Management Server please see the following article: TECH192608
Symantec Corporation is committed to product quality and satisfied customers. This Feature Request is currently being considered by Symantec Corporation to be addressed in a forthcoming version of the product.