I did this in Ubuntu 16.04 LTS. Configure Postfix to send mail from desktop to gmail. I wrote a post here.
Now that I'm using Archlinux, I want the same set up.
I installed postfix.
#pacman -Syu postfix
The config file for postfix is /etc/postfix/master.cf and /etc/postfix/main.cf.
Follow the steps in the link above.
Don't forget to restart postfix. Don't forget to protect the authentication file. Don't forget the postmap command.
Send a test mail.
$echo test | mail -s "test email setup" emailaddr