Problem:
Occasionally Outlook 2007 and 2010 will begin attaching a file called winmail.dat to outgoing emails, which is often annoying and confusing for both senders and receivers. The file is an Outlook-specific attachment format called TNEF, and therefore can’t easily be read by anyone not using Outlook.
Solution:
Microsoft offers a Fixit that purports to correct the problem by instructing Outlook to never attach winmail.dat. Additionally, turning off Rich Text in outgoing messages will fix the issue, but this eliminates the ability to apply text formatting.
Procedure:
- Visit http://support.microsoft.com/kb/958012 and download & run the Fixit there.
- If the Fixit does not correct the problem, follow the steps in the link above to manually make the changes.
- Alternatively, follow the steps here to turn off Rich Text sending in Outlook. (optional)
Limitations:
There are no known limitations to the method used in the Microsoft Knowledge Base article. The Rich Text disabling option will prevent custom text formatting, however as a consequence winmail.dat will no longer be attached to outgoing messages.