I set up in Outlook Express a filter that if the TO: or CC: did not contain my valid email address then it would be automatically deleted from the server and/or my inbox. (for some unknown reason sometimes one or two slip through, suspect some hidden characters in there).
Alternatively you can do a filter that everything that does not have a valid email address to go to some bulk folder which can be emptied once a week or so after review.
There is a lot you can do with filtering, I only recently started to work with this and it has been a blessing to get rid of 99% of the filth.
Maria