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 order to resolve this issue, the email needs to be re-sent in Plain Text format.
Method 1: Change the default message format
- On the Tools menu, click Options, and then click Mail Format.
- In Compose in this message format, click to select Plain Text, and then click OK.
NB: 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
- On the Tools menu, click Address Book.
- In Show Names From, click the Personal Address Book.
- Select the addressee that you want to set as plain text, and then click Properties on the File menu.
- 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: Change the specific contact format
- Open the recipient's record in the Contacts folder.
- Double-click the recipient's e-mail address.
- In the E-Mail Properties dialog box, click Send Plain Text only under Internet Format.
Method 4: Set the Outlook Rich Text Format Internet e-mail setting
- In Outlook 2003 and Outlook 2007, click Options on the Tools menu.
- Click the Mail Format tab.
- Click Internet Format.
- Under Outlook Rich Text options, click either Convert to HTML format or Convert to Plain Text format.