Open relay is a Simple Mail Transfer Protocol (SMTP) server that allows third-party relay of email messages. It is allows anyone to send messages through it, while it hides the source of the message being sent. An open relay makes sending large volumes of SPAM from unknown senders easy as it processes that is neither for nor form a local sender. Open relay is also known as an open relay server, insecure relay, third-party relay, open mail relay and spam relay.
Open relays does nothing to authenticate the sender of the email, this makes it vulnerable to address spoofing. Although, internet was essentially set up this way, spammers have often exploited this system. Since the internet has expanded exponentially, the potential for spam abuse has also expanded accordingly. Spammers have understood that by commandeering open relays, they can send out their spam. These unsolicited emails are delivered using address spoofing which seems like the messages came from a legitimate source.
Open relays have become highly unpopular due to the attack from spammers and worms. This is not a recommended configuration because it can be exploited by spammers. Servers with open relays are routinely blocked and/or blacklisted (check if your domain/IP is blacklisted). But the relay feature is part of all SMTP based servers which means if most of the modern email servers are unprotected, they are vulnerable to abuse. Spammers use automated tools to find vulnerable servers, an open mail relay will be eventually found.