open relay

open relay

Typically refers to an email server (SMTP server) that is configured to deliver any incoming mail to another mail server. In the past, open relays (open relay servers) were common, but today, most email servers block all email that does not originate with the customers of the service or employees of the company. The term may also apply to a proxy server that can be used by the general public. See spam relay and anonymous remailer.