Also referred to as an open relay server, an SMTP e-mail server that allows a third party to relay e-mail messages, i.e., sending and/or receiving e-mail that is not for or from a local user. Open relays make it possible for mobile users to connect to corporate networks by going first through a local ISP, which then forwards the message to their home ISP, which then forwards the message to the final destination. However, a downside of open relay technology is the proliferation of its usage by spammers looking to obscure or even hide the source of the large-volume e-mails they send.
Open relay also is called spam relay, third-party relay or insecure relay.
Contrast with mail relay.