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Recipients receive the Winmail.dat attachment

Wim van Sprundel
  • Updated

This article explains what you can do when recipients of your emails receive a winmail.dat attachment instead of the expected content or attachments.

 

Symptoms

Recipients report that an email you sent from Outlook contains an attachment named:

winmail.dat 

Sometimes the original attachments are missing or the formatting of the message is not displayed correctly.

 

Cause

Outlook can send messages in Rich Text Format (RTF). In doing so, Outlook may use Microsoft's Transport Neutral Encapsulation Format (TNEF).

Not all email programs can correctly process TNEF. In that case, the recipient may see a file named winmail.dat.

Recommended: use HTML as the default message format. HTML is the default format in current Outlook versions and is widely compatible with other email programs.

 

Solution in classic Outlook for Windows

Set Outlook so that new messages are sent in HTML by default.

  1. Open Outlook.
  2. Go to File > Options.
  3. Select Mail on the left.
  4. Find the setting Compose messages in this format.
  5. Choose HTML.
  6. Click OK.
Result: new messages are sent in HTML by default, so Outlook normally no longer uses TNEF/winmail.dat for regular emails.

 

Adjusting only one message

Do you want to adjust only the current message?

  1. Open a new message or reply to an existing message.
  2. Open the Format Text tab.
  3. Choose HTML.
  4. Send the message again.
Alternative: you can also use Plain Text. This is very compatible but does not support rich formatting, embedded images, or other HTML formatting.

 

Are you using the new Outlook?

In the new Outlook, HTML is the default normal message format. Microsoft now considers Rich Text a legacy format.

If a recipient still regularly receives winmail.dat files, the cause may not be the normal formatting setting of the new Outlook but rather an older Outlook client, a specific contact setting, or the Exchange configuration.

 

Problem persists

Does winmail.dat keep appearing?
Then contact your IT administrator. It may be necessary to check the TNEF settings in Outlook or Microsoft Exchange.

For organisations, TNEF can also be centrally disabled for external recipients or specific external domains.

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