This issue is with the sender's Outlook email client sending an email, usually with an attachment, in Rich Text Format (RTF). The recipient's email client is not able to decode the email properly and simply attaches the email in its entirety as "Winmail.dat."

The solution is to go into the sender's Outlook settings and change the "Compose messages in this format" to "HTML" or "Plain Text."  After that, you must delete the users Auto-Complete cache. The Auto-Complete cache will remember the RTF setting for that recipient and continue to use it.


*** Warning ***

Deleting the Auto-Complete list will clear out the emails that fill in when you type in the "To" field of an email. Do not do this if you have not been creating contacts for people. You will loose all of the email addresses in this cache.

Follow These Steps:

  • In Outlook 2007 and older, click on Tools then Options. In Outlook 2010, 2013 or 2016, click on File then Options.
         
  • Select the Mail tab.
         
  • Find the section called "Change the editing settings for messages."  Change the format to "HTML" or "Plain Text."
         
  • Scroll down to the "Send Messages" section, click on "Empty Auto-Complete List" and then click OK.
         
  • Finally, close Outlook and re-open it.