Has anybody else had their mail to the list mysteriously rejected recently due to attachments being disallowed by Lyris? I recently had this problem with an e-mail that seemingly DID NOT have an attachment. So we checked out the mail log and discovered the message had an attachment of type “application/ms-tnef” named “winmail.dat.”
A quick search of the January 2002 MSDN Library revealed that this hidden attachment is used by Outlook to contain font types and colors if you have your E-mail format set to “Microsoft Outlook Rich Text Format.” Not only that, the “winmail.dat” contains the path to your Outlook personal folders file (.pst) and your NT login name. Here’s the MSDN Library excerpt:
"The use of TNEF is commonly affected by settings in Outlook that are referred to as Microsoft Outlook Rich Text Format (RTF). Rich Text Format and TNEF are not exactly the same, but they are closely related.
A TNEF-encoded message contains a plain text version of the message, and a binary attachment that “packages” various other parts of the original message. In most cases, the binary attachment is named Winmail.dat, and may include the following information:
The formatted text version of the message (for example, font information and colors).
OLE objects (for example, embedded pictures and embedded Microsoft Office documents).
Special Outlook features (for example, custom forms, voting buttons, and meeting requests).
Regular file attachments that were added to the original message.
In addition to the previously listed information, the path to your personal folders (.pst) file and your log on name are embedded in the Winmail.dat file. Although this data is not explicitly exposed to the recipient, if the recipient opens the Winmail.dat file for editing in a binary or text editor, they can see the path and log on name. Note that the password information is not revealed. To ensure that the path to your .pst file or your log on name is not included in the Winmail.dat attachment, use the steps in this article to send messages that does not include the Winmail.dat file.