winmail.dat is an error when you are sending an attachment in Rich Text format:

When pdf's disapear after you send them or someone receives it as a
winmail.dat file your email account setup for sending format is NOT the only
place you have to turn off Rich Text Format for sending emails.  You must
also change the rich text format in the email address properties in your
address book for the person you are sending to.  double click on the email
address to bring up the email address properties and change it to let outlook
decide. The address book properties over ride what you choose in your email
account set up.
RobRose