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An open relay is a mail server which will accept emails from anyone and forward them on to any address.

This is a serious security problem, since anyone can make unauthorised use of an open relay to send unwanted emails. This places a heavy load on the server and can even crash it leading to the loss of legitimate email. From the point of view of the recipient of the unwanted email, open relays are a nuisance. Blocking all email from open relays often results in a sharp decrease in unwanted email with no loss of legitimate mail.

Open relays can be divided into two categories:

Single-stage

A single-stage open relay is a single computer which will accept an email and then connect directly to the recipient's mail server to deliver it. Most single-stage open relays are small servers hosting a few accounts.

Multi-stage

A multi-stage open relay is actually a series of mail servers one of which accepts emails and forwards them to another, which in turn forwards them to another and so-on until the last which tries to deliver them. Because the last server in a multi-stage open relay might be the smart-host for a large number of other, smaller, servers, blocking all multi-stage open relays risks the loss of some legitimate email.