Don't copy paste it, instead attach it using an attachment.
For Outlook Express: http://www.writersservices.com/www/w_attachments.htm
Thunderbird:
In composition video, hit attach and select the file that needs to be attached.
If you still need to copy and paste it, select the text in the word processor by clicking and dragging to the bottom of the text and then go to edit -> copy
Then, in the email window, go to edit->paste