Open relay (open mail relay) - a mail relay receiving and transmitting mail from an unlimited circle of people. As of 2008, open relays were recognized as an erroneous configuration of the mail system. Existing open relays are used for bulk spamming (spam), organizing denial of service attacks on other mail systems.
Open relays are usually included in the list of domains that send spam , regardless of whether there have been cases of spam from this domain or not.