i'm no expert, but I would speculate that you would call the model to get the relevant db data, return that as an array and then initiate the email sending in the controller.
For example, if a "forgot password" email is sent:
This is obviously pretty incomplete and happy to hear any alternative approaches.
// controller /user/forgotpassword
$email = $_POST['email'];
// fetch from model
$user_id = $this->findByEmail($email);
// call the email class / method inside genNewPassword