Thank you for answering my questions.
From what I have found online, it looks like McAfee VirusScan Enterprise has an e-mail scanner. There is a chance that it could be altering the attachment, or blocking it. If that is the case, Symantec Encryption Desktop will not be able to handle it properly.
* If you are certain that the attachment is safe, I would recommend attempting to open the attachment with the antivirus disabled.
Additionally, one of my previous suggestions may not have been followed all the way. Please try once more to access the e-mail / attachment with PGP Serivces disabled. Again, you can do this by right clicking the Symantec Encryption Desktop lock icon from the lower Right corner of your screen ( System Tray )
Please see the Image below for a screenshot of the area where you can exit PGP services.
After Exiting PGP services, and Symantec Encryption Desktop, please try to open Outlook and view the attachment. If it asks you to start PGP Services, please choose no. This is important so we can test to see if the problem is related to Symantec Encryption Desktop -- or if something else is modifying the attachment. Please send over a screenshot of Outlook during your testing. Thanks :D