Knowledgebase: Inbound Mail Flow
Winmail.dat file received – Unable to open
Posted by Ben McGilton, Last modified by David Szoke on 25 April 2018 08:09 PM

Winmail.dat Attachment – Unable to open

What is winmail.dat?

A winmail.dat is a file that can be referred to as a TNEF file. (Transport Neutral Encapsulation Format) and is a term for the format that Outlook utilizes.

This a file format that is not related to being filtered by Proofpoint Essentials that some customers receive and are unable to open.

How to prevent winmail.dat attachments being sent/received in Outlook:

  • Make sure that HTML or Plan Text is selected for “Compose Messages in this format” under “Compose Messages”
  • Make sure “Convert to HTML Format” or “Convert to Plain Text format” is selected when sending messages in “RTF to internet recipients” under “Message Format”
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