are you using microsoft outlook and sending winmail.dat file ?

here is a solution to stop sending those files and still continue sending your emails in HTML format. All other solutions say you need to change your default sending options to plain text but by using this method you do dont have to.

1) create a new (personal folder) - tools - options - data files - name the new personal folder "newbox"

2) After creating this folder, set it as your default folder

3)go back to  your inbox. you will now notice a new set of folders on the left navigation bar. Drag over your emails/folders to the new set of folders

4) go to file, import/export and backup all of your items such as your contacts, calendar, to do items ... save them to your deskstop

5) restart your computer 

6) once you have fully moved over your mail and backed up your contacts etc...go back to the data files menu and delete the one listed as "personal folder" this is your old data file. your new one will be called newbox and is set as your default

7) Start importing your contacts, calendar etc the same way you exported them.

and thats it. you done. now when you send emails they wont go out as winmail.dat !