Composing a document in a word processor may look like plain text, but it's not. However, if you go to File>Save As, and select 'Text' in the 'Save As Type' box, the document will be converted to ASCII and the file extension will be changed to .TXT.

However, if the finished document is going to be copied and pasted into a plain text email,  there is no reason to convert the file format into a .TXT file.

After pasting the text into an Outlook Express email for example, simply choose Format>Plain Text.

Yahoo mail has a Plain Text option located on the email form itself.

For AOL users, go to AOL Web Mail to send plain text.
 

For more, please visit this very informative site