Learn in the textbookAttacks on Networks and Communication Paths
DefinitionAn open relay (オープンリレー) is a mail server configuration that relays mail from anyone to anyone without restriction, never checking the sender or the recipient; it is also called third-party relay (第三者中継).
ExplanationOnce attackers find an open relay, they use it as a stepping stone for third-party relay. The fix is to configure the server to relay only mail sent by the organization's own users and to reject mail from outside addressed to outside. This setting still lets the organization's users send mail and still accepts mail from outside addressed to the organization.
AnalogyIt is like a parcel depot that sends out anything brought to it without looking at either the address or the sender.
Memory hook"Open" means open to anyone, and "relay" is the same word as in a relay race, where the baton is handed on to the next runner.