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What is email relay and why is it sometimes abused?

Email relay is when one mail server forwards messages to another on behalf of the sender. It is a normal part of how mail moves across the internet but can be exploited if a server allows open relaying, meaning anyone can send through it. Spammers abuse open relays to hide their identity or distribute bulk mail.

A properly configured relay server is like a secure harbor: only approved ships are allowed to dock and pass messages onward.