From MozillaZine Knowledge Base
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"):
--------------050804030404070201030211 Content-Type: application/ms-tnef; name="winmail.dat" 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.
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: