Why are people receiving winmail.dat attachments when I send them e-mails using Outlook?

The Microsoft Outlook e-mail program sometimes sends e-mails in the Transport Neutral Encapsulation Format (TNEF). Most other e-mail programs do not understand TNEF.

If your e-mail program doesn’t understand TNEF, instead of seeing the e-mail and/or attachment, you may only see an attachment named “winmail.dat” or “Part 1.2” that you cannot open. Also, sometimes you may receive a TNEF attachment with a generic name such as ATT00008.dat or ATT00005.eml instead.

Disabling TNEF in Outlook

The sender can avoid sending TNEF attachments by turning off TNEF in Outlook. When Outlook is configured to send e-mail in “Outlook Rich Text Format”, it may use TNEF. When it sends in “HTML” or “Plain Text”, it uses standard, compatible formats. In all versions of Outlook, you can disable TNEF completely:

  1. On the “Tools” menu, click “Options”, and then click the “Mail Format” tab.
  2. In the “Send in this message format” list, click “Plain Text” or “HTML”, and then click “OK”.