Spam report feedback loops

By monitoring spam reports from places such as spamcop, AOL's feedback loop, and Network Abuse Clearinghouse, the domains abuse@ mailbox, etc., ISPs can often learn of problems before they seriously damage the ISP's reputation and have their mail servers blacklisted.

Strong AUP and TOS agreements

Most ISPs and web e-mail providers have either an Acceptable use policy (AUP) or a Terms of Service (TOS) agreement that discourages spammers from using their system and allows the spammer to be terminated quickly.