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On the exchange server, go to global settings, Internet Message Formats, Default Properties, Advanced tab, for “Exchange Rich-Text Format” select “Never Use”.
This will prevent your Exchangeserver from using Rich-Text Format and creating winmail.dat files.
From TNEF’s website:
TNEF’s Enough allows Macs to read and extract files from Microsoft TNEF stream files. The files are usually received by SMTP based e-mail programs from Microsoft Exchange and Microsoft Outlook users. The SMTP based e-mail program will usually receive either a MIME attachment named “winmail.dat” or a MIME attachment with the type “application/ms-tnef.”
The file is a rich text (or MAPI) message that is sent from Outlook to Exchange. When Exchange sends the message to an outside server it writes the MAPI message as a MIME attachment. The unfortunate side effect of this plan is if the Outlook user has someone in their address book as a person who can receive “Rich Text” then the user will receive the TNEF file whether the user uses Outlook or not.
TNEF’s Enough is a freeware application