Resolve Undefined namespace Mailer CakePHP 3.0.3

I'm trying to create a Reusable Email provided by CakePHP 3 but even following the documentation but I get the following error:

use Cake\Mailer\Mailer;

Error:

  

Undefined namespace Mailer

     

Referenced namespace is not found.

I created the file in the same directory specified in the documentation

OBS: I'm using CakePHP 3.0.3

Link to documentation: Link