your ISP may have some filter set up, which you may access via their website. however, those bayesian filters don't work as well as one might like. block one IP range, and another will pop up in a few seconds. block one domain, and another will pop up. block one set of keywords, and they will soon change the keywords they use. in the end, you may wish to receive e-mails from registered or trusted recipients, which means no strangers at all.