TOSS 5 must be configured to prevent unrestricted mail relaying.

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.

Verify TOSS 5 is configured to prevent unrestricted mail relaying using the following command:

$ postconf -n smtpd_client_restrictions 

smtpd_client_restrictions = permit_mynetworks,reject

If the "smtpd_client_restrictions" parameter contains any entries other than "permit_mynetworks" and "reject", and the additional entries have not been documented with the information system security officer (ISSO), this is a finding.

Modify the postfix configuration file to restrict client connections to the local network using the following command:

$ sudo postconf -e 'smtpd_client_restrictions = permit_mynetworks,reject'