Notes from 2011...

Method 1: Set this relay server to use another server as a relay
yum remove sendmail
yum -y install postfix cyrus-sasl-plain mailx vim
systemctl restart postfix
systemctl enable postfix
vim /etc/postfix/main.cf
---------
relayhost = smtp.gmail.com:587
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_tls_CAfile = /etc/ssl/certs/ca-bundle.crt
smtp_sasl_security_options = noanonymous
smtp_sasl_tls_security_options = noanonymous
---------
vim /etc/postfix/sasl_passwd
---------
smtp.gmail.com:587 [email protected]:PASSWORD
---------
postmap /etc/postfix/sasl_passwd
chown root:postfix /etc/postfix/sasl_passwd*
chmod 640 /etc/postfix/sasl_passwd*
systemctl reload postfix
chkconfig --level 2345 postfix on


Test:
echo "This is a test." | mail -s "test message" [email protected]


Troubleshoot:
tail -f /var/log/maillog


Method 2: Set this machine as a relay server itself
vim /etc/postfix/main.cf
---------
myhostname = srvi-relay01.kimconnect.net
mydomain = kimconnect.net
mynetworks = 192.168.900.0/24, 10000.1000.1000.0/16, 127.0.0.0/8
---------
vim /etc/postfix/sasl_passwd
---------
smtp.gmail.com [email protected]:PASSWORD
---------