What is Winmail.dat and how do I stop it from happening

What is Winmail.dat and How Do I Stop it From Happening to My Emails


Winmail.dat is a file that gets created when email clients, usually Apple products like iPhones and MacBook’s, can’t parse or properly interpret specific email messages usually sent by Microsoft Products like Outlook and Office 365.


How can I stop Winmail.dat from happening?


The easiest fix is to stop Rich Text Format at an organizational level. 

Most, if not all, email clients can process HTML and Plain Text so we might as well default to those and turn of Rich Text because it doesn’t really serve a purpose.


How to Turn off Rich Text Formatting in Office 365

What is winmail.dat and how do I stop it from happening my emails

You will need to be an organizational admin

  1. Log into the Office 365 Portal
  2. Go to Admin
  3. Select the Exchange Admin Center
  4. Select – Mail Flow
  5. Remote Domains
  6. Click Edit on Default
  7. Set the ‘Use rich-text format” to NEVER
  8. Click SAVE


Turn off Rich Text Formatting in Outlook 2010 or new


  1. Go to File.
  2. Select Options.
  3. Go to Mail.
  4. In the Compose messages section,
  5. Select the Compose messages in this format dropdown arrow and choose HTML
  6. In the Message format section, select the When sending messages in Rich Text format to Internet recipients dropdown arrow and choose Convert to HTML format
  7. Select OK.


Prevent Winmail.dat Attachments in Outlook 2007 and Outlook 2003


  1. Select Tools Options.
  2. Go to Mail Format.
  3. Under Compose in this message format, choose HTML 
  4. Click OK.





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