What were the earliest anti-spam measures?

Early spam prevention relied on manual filtering, simple blocklists, and administrator rules. Messages from repeat offenders were blocked, and certain words or patterns were flagged.

The breakthrough came in 1997, when Paul Vixie created the first Real time Blackhole List (RBL), a shared, real time list that servers could query to block known spammers automatically.

It was like guards at the harbor finally getting a shared watchlist of banned ships, replacing scattered notes with a unified defense.