The term relaying can be interpreted as a relay function. This is understood to mean the unauthorized use of third-party mail servers. Mails are sent or forwarded via these servers without the owner noticing anything.
The sense of relaying lies on the one hand in the transmission of wrong, namely the foreign relaying address, on the other hand in the costs, which are charged to the foreign mail server.
This unauthorized relaying technique can be used, for example, to distribute spam or hoaxes without any direct information about the actual sender.
Relaying can be prevented by appropriate mail server configurations that do not allow any-to-any mails.