The Winmail.dat file is used to preserve Rich Text formatting. Outlook uses it when sending a Rich Text-formatted message. During transport, the content of the message may be changed, preventing the receiving client from being able to read the formatting instructions. In other cases, the receiving client does not use or recognize the winmail.dat file.

The data in a winmail.dat file is not usable. To resolve this issue, re-send the message in plain text format. The following methods can be used by sender to prevent sending Rich Text messages encapsulated in the Transport Neutral Encapsulation Format (TNEF).

Method 1: Change the default message format

The sender can change the format of the email messages that they send by using the following steps:  

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, the sender must set the option for the recipient in either the Personal Address Book or the recipient's contact record.

Method 2: Modify the recipient's entry in the Personal Address Book

The sender can 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.

Method 3: Set the Outlook Rich Text Format Internet e-mail setting

1. In Outlook 2003 and Outlook 2007, click Options on the Tools menu.
2. Click the Mail Format tab.
3. Click Internet Format.
4. Under Outlook Rich Text options, click either Convert to HTML format or Convert to Plain Text format.