Before allowing a user to add an endpoint in the control panel, Mandrill does a HEAD request at the endpoint to check if the route actually exists. They only let you add the route of the HEAD request returns 200.
With only the original post route, doing anything other than a POST on /email_processor returned an error, not letting you add the endpoint in Mandrill.
The two possibilities were changing ```post``` to ```match``` and adding ```via: [:post, :get]``` and then checking if the request was a get or a post in the ```create``` action, or doing what I did, create a new route pointing to a different action.