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. References:
* 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

The /etc/passwd file stores local accounts of the system. It is a readable text file and uses colons (:) to separate the fields. This file helps with converting user IDs to names (and back). It is fine that all users can read this file, but they should not be able to change fields. Therefore, the mostcorrect permissions for /etc/passwd are
-rw-r-r-, which means that only the owner (root) can write to the file, and everyone can read it. The
/etc/shadow file contains information about the system's users' passwords. It is owned by user root and group shadow, and has 640 permissions. The password is stored as a long string of characters, which is a combination of the hashing algorithm, optional salt applied, and the hashed password itself. Other users are not allowed to read the file directly, to prevent them from gathering hashed passwords of others. Therefore, the most correct permissions for /etc/shadow are -r--------, which means that only the owner (root) can read the file, and no one else can read or write to it. References: