Learn in the textbookAttacks on Networks and Communication Paths
DefinitionThird-party relay (第三者中継) is the act of an outsider with no connection to a mail server getting that server to relay (中継, forward a received message on to another server) their mail, using it to send messages.
ExplanationAttackers typically look for servers that accept mail from outside addressed to outside, route mail through them, and send spam in bulk. A device used as a pass-through for abuse like this is called a stepping stone (踏み台); because the recipients see the stepping-stone server as the sender, the attacker can hide their own identity.
AnalogyIt is like a company reception desk taking a stack of letters from a passer-by who is neither staff nor a visitor and mailing them out under the company's name. The complaints go to the company, not the passer-by.