Why do you not have the basic functionality of being able to forward tickets from Zendesk to external (non agent) emails?
The workaround using triggers and targets is frankly ridiculous and does not work as you then have to merge the response which comes in as a new ticket into the existing ticket which means the customer sees the entire ticket stream.
This is basic functionality that should be provided.
I note I'm not the first of your customers to request this - why have you not taken note of your customer's comments. To require us to purchase an app at significant cost to allow us to forward emails is not great customer service.