Debug Kit - Mailer Previews

I was wondering if in the 3.3.13 Version of CakePHP Version the Function of Mail Preview is already useable without the Plugin which is documented here (http://cakephp-mail-preview.readthedocs.io/en/latest/usage.html).

I tried the way described in the Debug Kit but it does not work throwing this exception:

Mailer class “User” could not be found.