Winmail.dat

From ITwiki

A winmail.dat file may appear as an attachment on e-mails received where the sender is using Microsoft Outlook and an e-mail format other than plaintext.

Using plaintext to avoid sending winmail.dat files

Microsoft Outlook allows users to automatically send plaintext only messages to certain recipients.

To do this:

• Open the Address Book and select the e-mail address you wish to modify.
• Open the properties for that e-mail address and click on the "Name" tab.
• Finally, check "Send email using plain text only".

Changing the format of a current e-mail is as simple as clicking on "Format" then "Plain Text".