Scenario: An email gets sent out and copies in the Queue Email Address. The Email router picks up the email and sticks it in the queue. Client wanted to automatically track the the email against the contact in the recipient address automatically (note: all contacts getting emails sent to already exist as contacts in the system). There was no way to do this using the workflow engine since the Recipient field and the Regarding field are two different types (partylist vs entityreference). I created a  plug-in to do the job. Register the plugin on pre-operation and this should do the job.