Postfix is a mail transfer agent (MTA), an application used to send and receive email. In this tutorial, we will install and configure Postfix so that it can be used to send emails by local applications only — that is, those installed on the same server that Postfix is installed on.
Why would you want to do that?
If you’re already using a third-party email provider for sending and receiving emails, you do not need to run your own mail server. However, if you manage a cloud server on which you have installed applications that need to send email notifications, running a local, send-only SMTP server is a good alternative to using a 3rd party email service provider or running a full-blown SMTP server.
In this tutorial, you’ll learn how to install and configure Postfix as a send-only SMTP server.
Source: How to Install and Configure Postfix as a Send-Only SMTP Server on Ubuntu 16.04 (Digital Ocean)