Why can't Erols or RCN block all of the junk email?

Our filters are an all-or-nothing proposition. To make sure that you don't get any junk email from (for example) aol.com, we have to block ALL email coming from AOL for ALL Erols users, whether it's junk or not. Since the overwhelming majority of the mail we receive from AOL is legitimate email, blocking AOL in this fashion would be throwing the baby out with the bathwater.

However, there are those domains who exist for no other purpose but to send out junk email. These are domains like nevwest.com, quantcom.com, and crushnet.com (just to name a few). With this bunch, the opposite is true. While some Erols users may have legitimate correspondence with people in these domains, the majority of the email sent from these folks to Erols customers consists of junk email. Therefore we can safely block these domains.