You can easily stop this message from appearing in your system tray by following these steps:

1. Press Windows key and then Escape to focus on the Start button.
2. Tab once to the Task Bar and press Alt-Enter to bring up the Task Bar Properties dialog.
3. Tab to the Notification Area Customize dialog button and hit Space.
4. Tab to the list of System Tray icons and press Y repeatedly until you land on "You Have New Email" , which, by default is set to "Always Show".
5. Right arrow once to the Behavior combo box , and arrow up to "Always Hide". Tab to Okay, hit Space and tab again to Okay and hit Space. This should permenantly suppress the email notification message in the System Tray. You can select "Always Hide" to suppress any other icon you don't want to appear in the System Tray.

the above steps also work for Windows vista, I don't know if they will work with windows 7. In order to find the customize dialog button in the taskbar properties, you need to have the hide inactive icons box checked.