In Open Relay, also known as an insecure relay or spam relay, is an SMTP e-mail server that allows a third party to relay e-mail messages. This means that unauthorized users, such as spammers, can send and/or receive e-mail that is not for or from a local user. Open relays make it possible for anyone to send mail through this type of server.
Email scripts, such as ones that are designed for use in mail forms on commercial web sites, may contain security holes that allow relaying through your system. It is important to check your scripts and specify who can use your scripts to send email. Read the installation information that accompanies your email script carefully to avoid unauthorized users sending emails with your script.
Spammers are the most frequent users of open relay servers, sending more than 1 million messages a day. Spammers send messages through somebody else’s mail server, thereby masking their real identity to the recipient. This is why most mail servers will not accept messages for relaying.
There are many tools available to test a mail relay to see if it will relay third-party email. Here’s a link, of many, that is used for testing open relay on mail servers, http://www.mailradar.com/openrelay/.