Insert your filter logic
At this point you need to determine what it is you are filtering on. The plugin provides you with the recipient email and the Input option & task.
The sample plugin validates that the recipient email address is in the Joomla #__users table.
Make sure you return false via the $event->setArgumant() call as shown to drop the recipient. If after processing all recipients for an email through this plugin the email has no recipients left the email will be dropped.