How to fix WinMail.dat attachment issues with Exchange

From time to time I will encounter an Exchange Server where the customer reports that attachments to emails they send out are being received as WinMail.dat files by some recipients. This hold true whether it’s an image file, a PDF file, etc.

The cause is quite simple: The Exchange server has an option to define whether or not to send attachments in “rich-text format”. If this option is enabled, this will cause attachments to be converted by some email clients into a WinMail.dat file. This is due to a methodology called “Transport Neutral Encapsulation Format” (TNEF) for sending messages across the Internet.

The solution is also quite simple: we just need to set the option on Exchange to “never use”. No rebooting of Exchange or the server is required after making this change.

Here are the detail steps (based on Exchange 2010):

  • Open up Exchange Management Console (EMC)
  • Drill down Organization Configuration –> Hub Transport
  • Click (to select) the Remote Domains tab
  • Right-click on Default, then click Properties
  • Click  (to select) the Message Format tab.
  • Under Exchange rich-text format section, click (to select) Never use
  • Click Apply and OK

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