Problem:

How to setup a Mail Relay in Postfix?
How to setup a SMTP Mail Relay in Postfix?

Solution:

Follow the guide below to configure Postfix to use outMail as a Internet Mail Relay.

This article already assumes your Postfix system is able to send emails externally and that you are already have a terminal connection to your server with the necessary access rights to manage and administor postfix.

First you need to create a password file for postfix to use. Create a file called /etc/postfix/relay_passwd and add the following lines to that file

mxXXXXXX.smtp-engine.com OUTMAILUSERNAME:OUTMAILPASSWORD

where OUTMAILUSERNAME and OUTMAILPASSWORD are the (case-sensitive) information provided when you set up your account.

To enable postfix to use the outMail service at the prompt type the following

postconf -e relayhost=mxXXXXXX.smtp-engine.com
postconf -e smtp_sasl_auth_enable=yes
postconf -e smtp_sasl_password_maps=hash:/etc/postfix/relay_passwd 
postconf -e smtp_sasl_security_options=noanonymous
postconf -e smtp_sasl_mechanism_filter=plain,login
postmap /etc/postfix/relay_passwd 

For the above changes to take effect the postfix service will need restarting (service postix restart)

Summary of server details