Purpose
Allows to open and edit your messages in an external text editor such as NEdit, emacs, etc...

Installation
Install as every other extension (install, quit, restart),
Open the compose window,
Select the menu View/Toolbars/Customize..., or
Right click on the toolbar and select Customize...,
Drag and Drop the new icon External Editor on your toolbar,
Open the extension window, select External Editor, click Options,
Set your editor (without path or with an absolute path)

Usage
Just click on the extension button, edit your message in your editor, save, close, and the message will be updated in the compose window.
The editor is launched in a separate thread and does not block the GUI.
External Editor has been tested on Windows (XP) and Linux. It should also work on Mac OS (X).