Ask the SA if the system is a documented mail relay, and if it is, this is not applicable.
If the system uses Sendmail, determine if Sendmail only binds to loopback addresses by examining the "DaemonPortOptions" configuration options.
# grep -i "O DaemonPortOptions" /etc/mail/sendmail.cf
If there are uncommented DaemonPortOptions lines, and all such lines specify system loopback addresses, this is not a finding.
Otherwise, determine if Sendmail is configured to allow open relay operation.
# find / -name sendmail.mc
# grep -i promiscuous_relay </path/to/sendmail.mc>
If the promiscuous relay feature is enabled, this is a finding.
If the system uses Postfix, locate the main.cf file.
Procedure:
# find / -name main.cf
Determine if Postfix only binds to loopback addresses by examining the inet_interfaces line.
Procedure:
# grep inet_interfaces </path/to/main.cf>
If inet_interfaces is set to loopback-only or contains only loopback addresses such as 127.0.0.1 and [::1], Postfix is not listening on external network interfaces, this is not a finding.
Otherwise, determine if Postfix is configured to restrict clients permitted to relay mail by examining the smtpd_client_restrictions line.
# grep smtpd_client_restrictions </path/to/main.cf>
If the smtpd_client_restrictions line is missing, or does not contain reject, this is a finding. If the line contains permit before reject, this is a finding.
If the system is using other SMTP software, consult the software's documentation for procedures to verify mail relaying is restricted.