How to prevent the Winmail.dat file from being sent to users
This article describes how either an Exchange Server administrator or end users can prevent the Winmail.dat attachment from being sent to Internet users when using the Microsoft Exchange Internet Mail Connector (IMC).
When an end user sends mail to the Internet from an Exchange Windows or Outlook client, a file attachment called Winmail.dat may be automatically added to the end of the message if the recipient’s client cannot receive messages in Rich Text Format (RTF). The Winmail.dat file contains Exchange Server RTF information for the message, and may appear to the recipient as a binary file. It is not useful to non-Exchange Server recipients.
Microsoft kb 138053
Select Tools | Options… from the menu.
Go to the Mail Format tab.
Under Compose in this message format:, make sure either HTML or Plain Text is selected.
Click Internet Format.
Make sure either Convert to Plain Text format or Convert to HTML format is selected underWhen sending Outlook Rich Text messages to Internet recipients, use this format:
Click OK again.