DNSBL
DNS-based Blocklist (also called RBL): a list of IPs or domains known for spam or abuse, published as DNS records for easy lookup.
A DNSBL (DNS-based Blocklist), also known as an RBL (Real-time Blackhole List), is a reputation list distributed via the DNS protocol. To check an IP like 192.0.2.5 against a list at example.dnsbl, a mail server queries `5.2.0.192.example.dnsbl` and treats any answer as a hit. Major lists like Spamhaus, SORBS, and SpamCop are used by mail servers worldwide to reject spam at SMTP time. Getting on a DNSBL can devastate email deliverability for a domain or shared hosting IP, so monitoring blocklist status is a routine part of operating a mail server.
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