The operating system must be configured to prevent unrestricted mail relaying. Rationale: If unrestricted mail relaying is permitted, unauthorized senders could use this host as a mail relay for the purpose of sending spam or other unauthorized activity.
Solution
If postfix is installed, modify the /etc/postfix/main.cf file to restrict client connections to the local network with the following command: # postconf -e 'smtpd_client_restrictions = permit_mynetworks,reject' Or you can manually add this line by editing the file: Example: vim /etc/postfix/main.cf Add this line: smtpd_client_restrictions = permit_mynetworks,reject Notes: This Benchmark recommendation maps to: Red Hat Enterprise Linux 7 Security Technical Implementation Guide: Version 2, Release: 3 Benchmark Date: 26 Apr 2019 Vul ID: V-72297 Rule ID: SV-86921r3_rule STIG ID: RHEL-07-040680 Severity: CAT II