An open-relay is a mail server which is not configured properly to prevent anyone on the Internet from using it to send e-mail messages. This is often unintentional, but is sometimes intentional. It is often the result of the organization owning the mail server not understanding security settings or just not caring about the consequences. Until the last few years, the default settings for most mail servers on Unix/Linux were as open-relays; many of these older machines are still running that way.

Spammers search the Internet for open-relays and then "program" them to send a continuous stream of spam. The owner of the open-relay typically learns of this via a flood of angry e-mails and then takes steps to shut it down. By then the spammer has moved on to another open-relay.