These instructions were written for Postfix 3, it might work with different major and minor version numbers but consult the Postfix documentation for your version to verify. Make sure to backup the existing files referenced below before you modify them in case you need to revert your changes.

1. Modify /etc/postfix/main.cf

In this file add following lines:

relayhost = smtp.sendmetric.com

smtp_sasl_auth_enable = yes

smtp_sasl_password_maps = hash:/etc/postfix/password

smtp_sasl_security_options = noanonymous

smtpd_sasl_authenticated_header = yes

smtp_use_tls = yes

2. Modify or Create /etc/postfix/password

In this file there should be a single line for each destination that you are relaying mail to. Below is the line for Sendmetric. Be sure to replace username and password with the SMTP user you created (please note you need to separate the username and password with a colon). For more information on SMTP users see the Getting Started Guide:

smtp.sendmetric.com  username:password

3. Create the password hash database for Postfix by running the following command on the CLI:

postmap password

4. Restart Postfix