Postfix is a Mail Transfer Agent (MTA) that can act as an SMTP server or client to send or receive email. There are many reasons why you would want to configure Postfix to send email using Google Apps and Gmail. One reason is to avoid getting your mail flagged as spam if your current server’s IP has been added to a blacklist.
In this guide, you will learn how to install and configure a Postfix server on Debian or Ubuntu to send email through Gmail and Google Apps.
Link to guide --> Configure Postfix to Send Mail Using Gmail