Setting up Postfix to relay through Google Mail

 Install postfix and required packages:

  # yum install postfix cyrus-sasl cyrus-sasl-plain cyrus-sasl-md5
  # chkconfig postfix on
  # chkconfig saslauthd on

Create SSL cert:

  # mkdir /etc/postfix/certs
  # cd /etc/postfix/certs
  # openssl dsaparam 1024 -out dsa1024.pem
  # openssl req -x509 -nodes -days 3650 -newkey dsa:dsa1024.pem -out mycert.pem -

keyout mykey.pem
  # ln -s mycert.pem CAcert.pem
  # openssl req -x509 -new -days 3650 -key mykey.pem -out mycert.pem

Create GMail authentication file:

  # vi /etc/postfix/sasl_passwd

Add the follow content:

  smtp.gmail.com username@googleapp.domain:password

Generate postfix sasl_passwd database:

  # postmap hash:/etc/postfix/sasl_passwd
  # chmod 600 /etc/postfix/sasl_passwd
  # chmod 600 /etc/postfix/sasl_passwd.db

Setting Postfix

  # vi /etc/postfix/main.cf

Edit main.cf

  inet_interfaces = all

  mydestination = localhost, $myhostname, localhost.$mydomain

  mynetworks = 172.30.10.0/24

Write the following contents to main.cf

  # SMTP relayhost
  relayhost = [smtp.gmail.com]:587
  
  ## TLS Settings
  smtp_tls_loglevel = 1
  smtp_tls_CAfile = /etc/postfix/certs/CAcert.pem
  smtp_tls_cert_file = /etc/postfix/certs/mycert.pem
  smtp_tls_key_file = /etc/postfix/certs/mykey.pem
  smtp_use_tls = yes
  smtpd_tls_CAfile = /etc/postfix/certs/CAcert.pem
  smtpd_tls_cert_file = /etc/postfix/certs/mycert.pem
  smtpd_tls_key_file = /etc/postfix/certs/mykey.pem
  smtpd_tls_received_header = yes
  smtpd_use_tls = yes
  
  # configuracion tls
  smtp_use_tls = yes
  smtp_sasl_auth_enable = yes
  smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
  smtp_sasl_security_options = noanonymous
  smtp_sasl_tls_security_options = noanonymous
  
  # alias de mapeo interno a externo
  smtp_generic_maps = hash:/etc/postfix/generic

Finally, restart postfix

  # postmap hash:/etc/postfix/generic
  # /etc/init.d/postfix restart

Credit to my senior Voo