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)

service postfix restart

Problem Solving

Below is a list of common gotchas.

SASL authentication failure

In your postfix logs you see message smilar to "mail postfix/smtp: warning: SASL authentication failure"

Double check you did the following

postmap /etc/postfix/relay_passwd
postfix reload 

SASL authentication failure: No worthy mechs found

In your postfix logs you see message smilar to "mail postfix/smtp: warning: SASL authentication failure: No worthy mechs found"

This usually indicates that the library SASL2 (Simple Authentication Secuiryt Layer) isn't installed.  Install the libsasl2-modules

sudo apt install libsasl2-modules

For CentOS, you may need to install cyrus-sasl and cyrus-sasl-plain.  You may also need to install cyrus-sasl-lib.

sudo yum -y install cyrus-sasl
sudo yum -y install cyrus-sasl-plain
sudo yum -y install cyrus-sasl-lib