When sending messages to users who do not use Outlook (including Web Messenger users), messages and/or their attachments may come through as MS-TNEF encoded winmail.dat files.
This will occur because the message uses the Outlook-proprietary Rich Text Format. This format is only compatible with Microsoft Outlook email clients.
This issue is not caused by any setting on the Universal Server or within PGP Desktop. The following knowledge base article from Microsoft details how to verify your Outlook and Exchange configuration to prevent this from happening.