Winmail.dat attachments

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This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).


The Microsoft Outlook e-mail program sometimes sends e-mails in the Transport Neutral Encapsulation Format (TNEF). Most other e-mail programs, including Thunderbird, 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.

Identifying TNEF attachments

While almost all attachments named "winmail.dat" are TNEF, you could receive non-TNEF attachments with names ending ".dat" or ".eml", or which are named or labeled "Part 1.2". In particular, AVG (an anti-virus program) can also add a "Part 1.2" attachment that contains the same information about the message having been scanned for viruses that it adds to end of the message body.

To verify whether or not you've received a TNEF attachment, highlight the message and click "View" -> "Message Source". Look for where the attachment begins; it looks something like the following (these are the "MIME headers"):

Content-Type: application/ms-tnef;
Content-Transfer-Encoding: base64

If you see "Content-Type: application/ms-tnef" then it's a TNEF attachment. If you see another "Content-Type", then it's likely something else.


You can use one of the programs below to open the TNEF attachment, or configure Thunderbird to use one of the programs automatically.

Online converter
  • Fentun Win9X, NT, Win2K and WinXP
  • Winmail Opener Windows 95X through to Windows 10
  • TNEF Linux (may be available in your distribution's repository)

Disabling TNEF in Outlook

The sender can avoid sending TNEF attachments by 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. There are two options for disbling TNEF:

In at least Outlook 2002 (a.k.a. Outlook XP) and Outlook 2003, if it's being used at a business, the following is recommended to ensure compatibility with corporate mail systems:

  1. On the "Tools" menu, click "Options", then click the "Mail Format" tab, and then the "Internet Format" button.
  2. Set "When sending Outlook Rich Text messages to Internet ..." to either "Convert to HTML format" or "Convert to Plain Text format".

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".

See also

External links