A quick list of commands to use gmail as a relay for postfix. This also works for office365. I used these instructions to easily get things set up :)
First step is to install postfix and mailx.
# yum install postfix mailx cyrus-sasl cyrus-sasl-plain
Next create /etc/postfix/sasl_passwd and add the following:
[smtp.gmail.com]:587 username@gmail.com:password
Now edit /etc/postfix/main.cf and add the following:
relayhost = [smtp.gmail.com]:587
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_security_options =
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_tls_CAfile = /etc/ssl/certs/ca-bundle.crt
Use postmap to hash the password so that postfix can use it.
# postmap /etc/postfix/sasl_passwd
Now enable postfix to start on boot and restart it.
# systemctl enable postfix
# systemctl restart postfix
We should now be able to send mail through gmail and we can test this through the following command.
# echo "Hello, World!" | mail -s "My First E-mail" recipient@domain.com
Voila! Now postfix is using gmail to send mail.