Add the following to the end of /etc/postfix/main.cf

smtpd_sasl_auth_enable=yes
smtpd_recipient_restrictions =
permit_sasl_authenticated,
check_relay_domains
broken_sasl_auth_clients = yes
smtpd_sasl_local_domain = $myhostname
sender_canonical_maps = hash:/etc/postfix/canonical

Set inet_interfaces to all in /etc/postfix/main.cf

inet_interfaces = all

edit the /etc/postfix/canonical file to rewrite the “from” address – if you have to deal with some spam filtering issues that dont like the from addresses

root@someserver.mydomain.com hostmaster@mydomain.com

generate the canonical.db file

postmap /etc/postfix/canonical

If needed, create an account to be used to authenticate and relay email

useradd mailrelay
passwd mailrelay

Restart the postfix service

service postfix restart