Occasionally clients receive unopenable attachments from users of Microsoft email programs. These attachments are of the type "application/ms-tnef", and often have the file extension DAT.This entry describes how to access the contents of these files.

Occasionally clients receive unopenable attachments from users of Microsoft email programs. These attachments are of the type "application/ms-tnef", and often have the file extension DAT. Other times the user encounters a blank message body with an attachment named WINMAIL.DAT. This problem results from Microsoft's implementation of rich text formatting for email also known as extended message formatting. Microsoft mail programs include two messages in any email. One version is rich text formatted and can be read only by Microsoft Mail clients. The other one is plain text and intended for non-Microsoft programs. In order to send these two messages as a single email, Microsoft uses a method called Microsoft Transport Neutral Encapsulation Format, or MS-TNEF. This a file called WINMAIL.DAT which is a MIME encoded copy of the rich text formated part of the message. When a Microsoft mail program receives this message it decodes this file and displays its contents. Unfortunately for non-Microsoft users any attachments are encapsulated within the MIME encoded part of the message. This means that the user can not access the attachment without decoding it.

For Mac users there is an application called TNEF is Enough which can decode these files.