The recent Love Letter virus infected millions of computer users. As you know, Love Letter spread quickly by accessing the user's address book and sending a copy of the virus to everyone listed therein.
Outlook users (and possibly users of other mail clients) might find it useful to have a dummy user in the address book to help detect future worms. By creating a fictitious user with a bogus email address, a user can make Outlook generate an onscreen error message about that bad address anytime it's used to send email, including when used by a virus or worm. Having such a bogus email contact won't stop a virus or worm, but it will alert you that something is accessing your address book without your approval. You can then contact your network security personnel to investigate.