WINMAIL.DAT attachments are sometimes created when sending mail using Microsoft Outlook. They can usually be ignored. They are basically just a copy of the email as an attachment.

They are created by Outlook to contain Transport Neutral Encapsulation Format (TNEF) information. This is essentially a copy of the email with the original formatting (fonts, colors, etc.), and it is useful only when the recipient is also using Outlook.

If you find that you frequently send WINMAIL.DAT attachments, and wish to prevent it, try checking the following (assuming you are using Outlook 2007 or 2010):
1)  Click the "File" tab, then "Options".
2)  Click "Mail" in the left pane.
3)  Ensure that "Compose messages in this format" is set to "HTML" or "Plain Text". In particular, ensure it is NOT set to "Rich Text". The default is "HTML". Be aware that "Plain Text" sends (literally) plain text with no special formatting. This is less attractive, but easier to read on some devices.
4) Sometimes, WINMAIL.DAT creation is tied to a particular contact in your address book. If you continue to have issues, contact the Campus Support Center at campussupport@atu.edu.