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Microsoft Outlook and Exchange Server use a special data-encapsulation method called TNEF -- short for Transport Neutral Encapsulation Format -- to send messages across the Internet.
TNEF-encoded messages consist of a plaintext version of the message. They also include a binary attachment called WINMAIL.DAT with any non-plaintext data, which contains a richly-formatted version of the message, any embedded OLE objects, Outlook-specific features, such as custom forms, and conventionally attached files.
Typically, the only way to know what's inside a given WINMAIL.DAT package is to read it through Microsoft Outlook or another client that supports deciphering TNEF.
However, developers and mail administrators who want to decipher a WINMAIL.DAT package can now do so without having to use Outlook, thanks to a third-party program called TNEF View.
TNEF View (free trial available; single-user license is $68) lets you take any WINMAIL.DAT attachment and decode it to reveal its contents. The program does not require Outlook or any other MAPI client application installed to work.
All message bodies, attachments and properties can be deciphered through the program, and can be viewed in a variety of ways: a hex dump, plaintext, RTF (for richly-formatted messages), metadata dump, and a list of all embedded OLE objects. OLE objects can also be extracted and launched from their host applications, if any are present.
In addition to deciphering TNEF messages, the program can also unpack .MSG-formatted files from within MAPI stores. Deciphered messages can be saved to disk, along with all of their attendant components, and the program can also handle the presence of nested attachments (messages inside messages, etc.).
Unfortunately at this time the program has no easy way of extracting message data from messages saved in Outlook Express without some binary-level hacking. (This process is documented in the program's help, however.)
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.
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If you don't need all the diagnostic data and just want a quick way to get the info out of a WINMAIL.DAT attachment, there are a couple free utilities that we've used that worked fine:
There is a Linux utility that does a fine job fine as well:
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