The email address
at the machine named localhost
On most machines (especially Unix machines), localhost is defined somewhere
to point to the loopback network address (127.0.0.1). This is certainly
not universal, and many non-Unix systems (MicroSoft home OSes)
don't understand this mapping correctly.
'root' is the login of of the system administrator of a Unix system. While
this is fairly universal, some places the login is changed to some other
(all that really matters is the uid 0). Some poorly written programs assume
that the user root is always the administrator - this is not the case.
When this email address is harvested by a spammer, it very rarely means
anything special to him or her. If the spammer is sending out the email
directly from his or her machine (running a Microsoft OS), root@localhost won't go anywhere at all. The email will fail to be delivered - one of thousands that points to an invalid email address. Maybe, if the spammer is silly enough to have an smtp server on his or her own machine, they may get one copy of the spam. Whoop de do.
If the spammer is using a relay of some sort that is on a Unix machine, instead the administrator of the relay will get email. 'root' often gets tons of email from automated processes and a fair chunk of it is killed without a glance. This is also typically the case for advertisements to root. At best, the admin will look at the headers and notice he or she has an open relay and fix it. At worst,you (the person who has put 'root@localhost' for harvesting) has just made the job of a system admin somewhere that much tougher and less rewarding.