This tutorial is going to show you how to set up Postfix SMTP relay with Mailjet. There are some hosting providers or ISPs that block port 25 to as a way to control email spam. You can bypass port 25 and send email to outside world with SMTP relay because it uses port 587. With SMTP relay, your own email server doesn’t directly send email to the destination email address. Instead, there’s an intermediate mail server, otherwise known as smart host or relay host , that sends email on your behalf. Your email server communicates with smart host on port 587, then the smart host talks with the recipient’s mail server on port 25.