What were the earliest anti-spam measures?

Before the mid-1990s, spam prevention was mostly manual work. Sysadmins wrote rules to block specific senders, flag certain words, or filter by subject line. Each mail server fought spam alone, using its own blocklist of known offenders. If a spammer changed their address or server, the whole process started over.

The breakthrough came in 1997, when Paul Vixie created the Mail Abuse Prevention System (MAPS) and launched the first Realtime Blackhole List (RBL). Instead of each server maintaining its own blocklist, MAPS RBL was a shared, centralized database that any mail server could query in real time. If a sender's IP address appeared on the RBL, the receiving server could reject the email before it even hit the inbox.

This was the moment spam filtering became collaborative. One spammer could be reported once and blocked everywhere that used the RBL. It scaled in a way manual filtering never could.

MAPS RBL wasn't perfect (false positives were common, and getting delisted could take weeks), but it proved the model. By the late 1990s, other RBLs appeared: Spamhaus launched in 1998, SpamCop in 1998, SORBS around the same time. Each had different criteria for listing IPs (open relays, spam traps, complaint volume), and mail servers could check multiple RBLs before deciding whether to accept a message. Modern blocklists still work on the same principle Vixie established in 1997: shared intelligence, real-time queries, collaborative defense. Today's systems are more sophisticated (they track domain reputation, not just IPs, and use machine learning to spot patterns), but the core idea hasn't changed. One bad actor, reported once, blocked everywhere.

And if you're sending email today and you've ever wondered why your IP or domain ended up on a blocklist, you're dealing with the direct descendants of MAPS RBL. Worth checking where you stand with our free blocklist checker.