Mail filtering rule not working

To try to stop the huge, annoying flood of virus/worm messages with which I’ve been deluged lately, without having to download them, I’ve attempted using the filtering rules from the Dreamhost panel. One of the rules I added says to “delete emails with mailserver in the to”. It doesn’t seem to be working; I’m still getting messages with “To:” lines such as ‘“Inet Client”’, which have the substring “maildomain” and should hence be caught. Am I doing something wrong?

– Dan