Few notes about how to block a specific domain to send out emails
sh
To help in cutting down the number of spam mails currently getting through a specific domain
# vi /etc/postfix/blacklisted_domains
# cat /etc/postfix/blacklisted_domains
mydomain.com REJECT
# postmap /etc/postfix/blacklisted_domains
# postconf -e 'smtpd_recipient_restrictions = check_recipient_access hash:/etc/postfix/blacklisted_domains, permit'