Sometimes you may receive an email with a winmail.dat attachment. This file is sent from people using Microsoft Outlook as their email client and they are sending emails in Microsoft Outlook Rich Text Format instead of Plain Text format.
How to Configure Outlook NOT to send Winmail.dat attachments
To Turn off Rich Text sending for messages in Microsoft Outlook:
1) Click on Tools
2) Click Options, and then click the Mail Format tab.
3) In the Send in this Message Format list, select Plain Text, and then click OK.
This will set your default sending method to Plain Text, which will lose your special formatting options with fonts, colors, etc.
However everyone, no matter what email program they are using, will now be able to receive your email with no problems.