A DNS-based blocklist (DNSBL) lets a receiving server check whether a connecting IP is listed, typically via a fast DNS lookup, and reject or heavily filter mail from listed sources. Major public blocklists include Spamhaus and SORBS, among others, each with its own listing criteria.
Listings usually result from a spike in complaints, spam-trap hits, or a sudden volume/reputation anomaly, not a single bad message. Delisting processes vary by list operator and can take anywhere from hours to weeks depending on severity and history.
Shared IP pools mean one sender's bad practices can get an entire pool listed; this is one reason lane isolation and dedicated IPs matter for senders with strict deliverability requirements.