use Cake\Validation\Validator;

...

$validator = new Validator();
$validator
    ->validatePresence('email')
    ->add('email', 'validFormat', [
        'rule' => 'email',
        'message' => 'E-mail must be valid'
    ])
    ->validatePresence('name')
    ->notEmpty('name', 'We need your name.')
    ->validatePresence('comment')
    ->notEmpty('comment', 'You need to give a comment.');

$errors = $validator->errors($this->request->data());
if (!empty($errors)) {
    // Send an email.
}