Remediation Steps

Follow the steps below to remediate this finding on Linux (Debian/Ubuntu).

1. For Postfix, restrict relay access in /etc/postfix/main.cf:

   smtpd_relay_restrictions =
   permit_mynetworks,
   permit_sasl_authenticated,
   reject_unauth_destination

2. Set your trusted network:

   mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128

3. Restart Postfix:

   sudo systemctl restart postfix

4. Test with: telnet localhost 25 and attempt to relay to an external address.