Starting postfix results in warning: /etc/postfix/main.cf: unused parameter. What can I do to fix this?

Make sure you are always using the most current version of Postfix. This error can occur if you are using a version of Postfix that does not support a certain directive or parameter.

For example, Postfix 2.9.3 does not support the smtpd_relay_restrictions directive. That directive was introduced in Postfix 3.10. Prior to version 3.10, relay permissions were combined into smtpd_recipient_restrictions.