Sending mail to a destination via a third-party mail server or proxy server in order to hide the address of the source of the mail. When email servers (SMTP servers) are used, it is known as an "open relay" or "SMTP relay," and this method was commonly used by spammers in the past when SMTP servers were not locked down. Today, most spam relay is provided by proxy servers and botnets. See
spam,
spam filter,
botnet,
open proxy server and
MAPS.