I found Sven_Franks last comment interesting. It sounds like it's maybe just a problem with Microsoft Outlook 2013 64-bit edition then?
@bolsover - I would suggest a few things:
1) Check your Outlook trust center to see what other add-in's are loaded in Outlook.
2) Can you also please check the Windows registry for these values to see what they are set to? They should be set to 1 for Email Encryption support
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\PGP Corporation\PGP]
"PGP_INSTALL_MAPI_PLUGIN"=dword:00000001
"PGP_INSTALL_LSP"=dword:00000001
"PGP_INSTALL_MAPI"=dword:00000001
You should also have the following:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_DLLs
Check and see if PGPmapih.dll is listed in there. If you have any security software enabled. Try disabling or possibly uninstall that to see if that helps.
3) To put Symantec Encryption Desktop in full debug mode go here:
http://www.symantec.com/docs/HOWTO64205
Right click PGP tray afterwards and you can select the Debug Logging option. You can then choose to check all the options such as Debug, Function entry/exit, Raw data, etc. set proxylib to 00F00 and TLS to 0700. This will allow you to capture much more verbose logs form the Desktop client. If you would like, I can share an secure file storage location for you to share the logs with us of your testing. I would like to confirm if this is already a known issue or if we need an additional bug logged to track this problem on Outlook 2013.
As others have mentioned on this forum, I think it's important to note that we do not currently have support for Outlook 2013. But it would be good to do due dilligence and also provide best effort support in this case.