Why do some of my messages have a “winmail.dat” file attached in addition to or instead of the normal attachment(s)?

This has to do with Microsoft Outlook and Exchange mail software, please see the following documents:

• How to Prevent Winmail.dat Attachments from Being Sent in Outlook:
  http://email.about.com/od/outlooktips/qt/Prevent_Winmail_dat_Attachments_from_Being_Sent_in_Outlook.htm
• Disabe Winmail.dat Stubbornly Going to Recipients No Matter the Default:
  http://email.about.com/od/outlooktips/ss/wt121705a.htm