open relay
also: mail relay, SMTP relay
An email server that accepts and forwards messages from any sender to any recipient without authentication or restrictions, allowing spammers to use it to send unsolicited mail.
An open relay is a mail server (SMTP server) configured with insufficient access controls that permits unauthorized users to send emails through it. This differs from a properly configured mail server, which authenticates users and restricts relaying to legitimate recipients.
Open relays became a major problem in the 1990s and early 2000s when spammers would exploit them to send bulk unsolicited email (spam) while masking their true identity. Since the mail server forwarded the messages, the spam appeared to originate from that server rather than the attacker.
Example: An unprotected SMTP server listening on port 25 might accept a connection from anyone and relay mail to external domains without checking credentials. A spammer could connect and send thousands of emails to random recipients through it.
Modern mail servers prevent this through SMTP authentication (requiring a username/password), IP whitelisting, and restricting relay permissions to authorized users only. Open relays are now rare due to widespread awareness and server security improvements.