For a standalone version of PGP Desktop:

1. Close Symantec Encryption Desktop (formerly known as PGP Desktop) and Exit PGP Services from the PGP Tray icon in the Taskbar.
2. Click Start and select Run.
3. Type %appdata% and press Enter.
4. Open the PGP Corporation > PGP folder.
5. Inside the PGP folder is a file called PGPprefs.xml, right-click the file and open it with WordPad.
6. Find the passthroughInboundSMIME key and change the value from false to true and save the file.
7. Restart the PGP Tray (Start > Programs > Startup > PGP Tray). 

For a managed PGP Desktop:

1. Go to Consumers > Consumer Policy and select the desired policy

2. General and select Edit.

3. Edit the xml preferences and type into the Pref name: passthroughInboundSMIME type: boolean value: true.

4. Re-enroll the PGP Desktop client.