Winmail.dat attachments are included in received e-mail messages in Outlook

This is an extract from Microsoft's KB article 278061 Read the full article here.
SYMPTOMS: When you use Microsoft Outlook to send messages through the Internet with attachments and you use the Microsoft Outlook Rich Text format, some recipients report that the message includes an additional file called the Winmail.dat file. The Winmail.dat file is usually very small, but you cannot open it in the message. The original message attachment is not always separate from the Winmail.dat file attachment, and may be included in the Winmail.dat file attachment.
CAUSE: This problem occurs because the Winmail.dat file is used to preserve formatting that the sending client includes in the message, but the receiving client does not recognize the Winmail.dat file. In Outlook, the Winmail.dat file includes Rich Text Formatting (RTF) instructions. This type of formatting is used with the Microsoft Outlook Rich Text format and when you use Microsoft Word as your e-mail editor.
RESOLUTION: To resolve this problem, use the plain text format. To do this:
1. On the Tools menu, click Options, and then click Mail Format.
2. In Compose in this message format, click to select Plain Text, and then click OK.
NOTE: To send to certain recipients that use RTF format and others recipients that use plain text format, you must set the option for the recipient in either the Personal Address Book or the recipient's contact record.
Use the following steps to remove the RTF format from the recipient attribute in the Personal Address Book:
1. On the Tools menu, click Address Book.
2. In Show Names From, click the Personal Address Book.
3. Select the addressee that you want to set as plain text, and then click Properties on the File menu.
4. In the SMTP-General tab, click to clear the Always send to this recipient in Microsoft Exchange rich text format check box, and then click OK.
Use the following steps to set plain text in the recipient's contact record:
1. Open the recipient's record in the Contacts folder.
2. Double-click the recipient's e-mail address.
3. In the E-Mail Properties dialog box, click Send Plain Text only under Internet Format.

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