1. Click on "Compose," and enter the address and subject line.
2. Before composing the message, click on "Tools" (next to "Copy message to
sent folder").
3. On the drop-down "Tools" menu, the last entry is a toggle switch to turn
the rich-text editor on and off. If you see "Rich-text editor ON" it means
you will be sending the message as plain text (it is giving you the option
to turn it on, which you don't want to do). If you see "Rich-text editor
OFF," click on it to toggle it to the off position.

Note: If you see a tool bar with formatting functions including bolding
(B), paint cans, etc. you are not sending the mail as plain text. Go into
the "Tools" menu and click on "Rich-text editor OFF" so it is in plain text
mode. The tool bar will disappear when you do that.

http://helpdesk.rootsweb.com/listadmins/plaintext.html#hotmail