Sounds like you need to edit /etc/aliases (depending on your server setup, should work for most machines) and change the line that looks something like
root: jenny
to
root: jenny@ordinary-email-address
Isn't it better to create a .forward file in Jenny's home dir, the contents of which (on the first line) are the external address.
Can't think why it is better that way (apart from the user not needing to be su to change the address later) but it's the way I've always done this in the past.