Did you search the rest of the million or so articles in the MSKB?
Winmail.dat attachments are included in received e-mail messages in Outlook
2002 or in Outlook 2003:
http://support.microsoft.com/kb/278061/en-us

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Milly Staples [MVP - Outlook]
Post all replies to the group to keep the discussion intact. All
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reading.
After furious head scratching, Steve asked:
||| When i send from Outlook it sends a attachment called winmail.dat I
||| do not think it is anything BUT how do i get rid of it sending it
[quoted text clipped - 8 lines]
|| That only applies to Microsoft Exchange Client 4.0 I using a sigle
|| user version