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.
How do spammers take advantage of the concept of email relaying?
Elsewhere on the Web:
The MAPS Relay Spam Stopper is a database of open mail servers relaying spam.