See Email received from a sender using Outlook includes a Winmail.dat attachment.
Plain text is always good (in my opinion) and should be reliable. If you must use rich text, try HTML.
You receive an email message that contains a winmail.dat
attachment. This issue may occur if all of the following conditions
are true: The email message is sent to you by someone using Microsoft
Outlook. The format of the message is Rich Text format (RTF).
This issue is more common when the email message is sent to you over
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, ask the sender to 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
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: 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. 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: 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
The sender can use the
following steps to set plain text in the recipient's contact record:
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
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