Blocklist of known spammer IP and ISP's

Use as any other blocklist, an example for a sendmail .mc file :

FEATURE(dnsbl, `', `"550 Mail from " $&{client_addr} "(use your favorite message) "')dnl

Any spamming ip will return

Although any message may be returned in the SMTP rejection message, a message hinting to an explaining page might be beneficial, in order to have competent mail-admins figuring out the reason . One example might be "Blocked due to spam(association), see http://(location of message)/refused.html"

The list itself is built by real spammers and their associated provider ranges. And yes, it's huge ( and growing ).