When an email service provider or a spam filter singles out an IP address on suspicion of phishing or spamming, it is called blacklisting.
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There are different blacklisting organizations such as Spamhaus, SORBS and Spamcop founded with the primary purpose of assessing, managing, and reporting any spam related activity across the internet. ISPs rely on their reputation and prowess to identify server IPs that have the potential to be problematic.
The addresses that end up on this blacklist undergo additional screening for spam, or their messages are blocked altogether. Given that over 67% of all emails sent or received are spam, it’s easy to appreciate why these strict measures are in place. By using complex filters and verifying the legitimacy of each mail, the ISPs protect their clients from harm. Blacklists are critical in the fight against spamming.
Blacklists are created from a pool of public information together with the ISPs own compiled data. Different algorithms make up the spam filters, picking on different indicators that might suggest a message is harmful. For instance, if an email is sent and repeatedly flagged for spam, it automatically gets the IP address blacklisted.