So I relaxed and I figured out how to relay emails using an Google app (GMAIL) account.
After reading a lot of post, I realized that everybody was missing a part of the problem.
Some people shows you how to create client certificates for postfix when you do not need them.
Some people tell you to get the root CA certificates when you already have them.
However the most important part is to have all the required packages to make it work.
For Centos 5.3
Verify package installed
# rpm -qa |grep postfix
# rpm -qa |grep sasl
# rpm -qa |grep openssl
Copy root CA certificates. Postfix needs to know the location of root CA.
# cp /etc/pki/tls/certs/ca-bundle.crt /etc/postfix/cacert.pem
Create file that stores GMAIL user and password.
# vi /etc/postfix/sasl_passwd
# Relay all e-mail via GMail.
relayhost = [smtp.gmail.com]:587
# SASL authentication
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_tls_security_options = noanonymous
smtp_sasl_mechanism_filter = login
smtp_use_tls = yes
smtp_enforce_tls = no
smtp_tls_CAfile = /etc/postfix/cacert.pem
smtpd_tls_received_header = yes
tls_random_source = dev:/dev/urandom
Finally, restart postfix.
Enjoy it ...