Glossary
Open Relay
An open relay is a mail server configured to forward messages from any sender, to any destination, without checking whether the sender is actually authorized to route mail through it. This was, historically, one of the most common and consequential authorization failures in email infrastructure specifically: a single misconfigured relay could be used by anyone to send mail that appeared to come from a domain they had no relationship to.
The lasting lesson generalizes well beyond email: any system that forwards, relays, or acts on a caller's behalf needs to verify the caller is actually authorized to use it for that purpose, rather than assuming a request reaching the relay at all implies it's legitimate.