I created a new controller action called "AddEmailToCustomerModel"
But I got an exception after trying to call the route:
Exception #0 (Magento\Framework\Exception\RuntimeException): Type Error occurred when creating object: Fekete\Newsletter2Go\Controller\Index\AddEmailToCustomerModel\Interceptor
So I removed the name Model from the action name and its filename, and it works now!
Why does it not work if there is Model in the name?
And are there more reserved words we should know about?