Enabling the plugin
Before you enable the JMailQ system plugin, the JMailQ options must have been opened and saved. If you did not do any configuration of the JMailQ options you must go and do that now or the plugin will not operate properly.
Once you have configured the JMailQ options and set up the delivery Cron task, you are ready to enable the plugin. Do this through the Administrator Extensions->Plugins interface
Once you enable this plugin all J! emails should go through the JMailQ system.
A note on plugin ordering. JMailQ is designed to inter-operate with the AngkorMailer component. The AngkorMailer component intercepts the Joomla Mail in the same way JMailQ does. In order for this to play well with JMailQ, the AngkorMailer system plugin must be higher in priority than the JMailQ plugin. Check this and adjust as necessary in the Administrator Extensions->Plugin interface.
It is also highly recommended that the JMailQ system plugin has the highest priority (subject to AngkorMailer as above).