I am seeing a number of Eudora attachments on my machine that are identified as winmail...I cannot open them. What are they?

E-mail prepared using the Microsoft Outlook e-mail client or Microsoft Exchange Server may include a file attachment called winmail.dat at the end of the message. When Rich Text Format (FTF) or HTML are used to compose the mail, Outlook automatically generates this file, which contains formatting information that Outlook needs to display the message correctly. By the way, this proprietary formatting is called Transport Neural Encapsulation Format (TNEF).

Although Outlook understands this information, non-Microsoft e-mail clients such as Eudora cannot process it, with the result that the file either cannot be opened or contains garbage characters.

Although may winmail.dat attachments are harmless and simply contain formatting information, they can also provide a great place for viruses to hide! So a great rule of thumb is: Do not attempt to open these files, or any other attachment, unless the user is know to you or the file is expected. For a handy review of E-mail best practices, including how to handle attachments, go to this link: E-Mail Handout.