In this article, I will show you how to set up Postfix as a send-only SMTP server on CentOS 8. I will first show you how to do it for a single domain, then you can apply the steps for multiple domains if you need to.
Use Case
You have a website/web application that needs to send transactional emails to users (such as password-reset email). Most likely, there’s no need for users to reply to these emails, or if they reply, the reply emails will be sent to your dedicated mail server. In this case, you can set up a send-only SMTP server on the web server using Postfix, which is a popular SMTP server software.
Prerequisites
In order to send emails from your server, port 25 (outbound) must be open. Many ISPs and hosting companies such as DigitalOcean block port 25 to control spam. I recommend using Hostwinds, because it doesn’t block port 25 (outbound). Once you have a Hostwinds server, install CentOS 8 on it, and follow the instructions below.
Setting up Postfix send-only SMTP server for multiple domains isn’t difficult actually. First, we need to configure it for one domain, then set it up for multiple domains.
Read full tutorial here: https://www.linuxbabe.com/mail-server/postfix-send-only-smtp-server-centos-8
