Open relay

An open relay (also called open mail relay) is an SMTP server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just e-mail destined to or originating from known users. This used to be the default configuration in many mail servers; indeed, it was the way the Internet was initially set up, but open mail relays have become unpopular due to their exploitation by spammers and worms. Many relays were closed, or were placed on blacklists by other servers.