By monitoring spam reports from places such as spamcop, AOL's feedback loop, and Network Abuse Clearinghouse, the domain's abuse@ mailbox, etc., ISPs can often learn of problems before they seriously damage the ISP's reputation and have their mail servers blacklisted.
There are various sites who maintain POC databases, such as Network Abuse Clearinghouse (by name), Abusix (by number), and more.
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