SMTP stands for Simple Mail Transfer Protocol, which is a standard for sending and receiving email messages over the Internet. An open relay is a SMTP server that is configured to allow anyone on the Internet to send email through it, without verifying the identity or the origin of the sender. This can be exploited by spammers and hackers to send large volumes of unwanted or malicious email, while hiding their true identity and location. Open relays are considered a security risk and a source of spam, and are often blocked or blacklisted by other email servers. To prevent open relays, SMTP servers should be configured to only accept email from authorized users or domains, and to reject or filter email from unknown or suspicious sources. Reference:
Open mail relay - Wikipedia
What is Open Relay? - Definition from Techopedia
What is an open relay and how do I close one? - Validity Help Center
LPI Linux Essentials: 1.5 Security and File Permissions: 1.5.3 Network Security LPIC-1: System Administrator: 102.5 Implement basic network security: 102.5.1 TCP Wrappers