***** INSTALLING POSTFIX ON DEBIAN/UBUNTU ***** ********* Part 1 ************* Require HELO as per RFC $sudo sh -c "echo smtpd_helo_required = yes >> /etc/postfix/main.cf" $sudo service postfix restart Try From the Server, telnet localhost port 25 $telnet localhost 25 Then type the following pressing enter after each line: mail from: manager@192.168.0.4 You should be prompted to require HELO first ************* Part 2 *************** Require a fully qualified domain name from the sender to approve email delivery sudo sh -c "echo smtpd_sender_restrictions = reject_unknown_sender_domain >> /etc/postfix/main.cf" telnet your server's port 25 helo bla mail from: sgfg@sfgdfgdfgdfgdfgdfgkdfjgkdfgjkdfgd.com rcpt to: afnog@localhost ************* Part 3 *************** Require the sender to have a Fully Qualified Domain Name on the Helo sudo sh -c "echo smtpd_helo_restrictions = reject_non_fqdn_helo_hostname >> /etc/postfix/main.cf" helo bla mail from: sdfd@google.com rcpt to: afnog@localhost