What is an open mail relay?

 

An 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, and not just mail destined to or originating from known users.[1][2][3] This used to be the default configuration in many mail servers; indeed, it was the way the Internet was initially set up. However, open mail relays have become unpopular due to the abuse of their use by spammers and also the spread of worms. Many relays were closed, and some were even placed on the blacklist by other servers.