Sender Filtering in Exchange 2003

Running exchange 2003 on a windows 2003 server. Company is a bit too cheap to spring for a decent spam filter so I'm just using the sender filtering that's built into exchange 2003. Well I've got a block set up for "*.@stderr.*.com" for example because we keep getting tons of spam from random addreses at a doman name that is "[insert random address]@stderr.[insert random item here].com" so I figured that blocking "*.@stderr.*.com" should do the trick but I still get e-mail from these addresses. Any ideas? Thanks.