Open Relay

When the Internet was young, people trusted each other (or there simply weren't too many of them) and everybody was allowed to use everybody else's SMTP server. The server would take the email message and forward it to the intended recipient. Spammers exploited that openness. They sent their unsolicited 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 today. Servers that will accept such messages are known as open relays.