Laravel unique email validation is not working on different character case in mongodb

I have a registration form with unique email validation. When I enter different character case, emails does not apply unique validation.

example@gmail.com, Example@gmail.com, eXample@gmail.com: Laravel unique validation is failed.

example@gmail.com, example@gmail.com, example@gmail.com: Laravel unique validation is success.

Please check my code below and correct me. Email is storing as lowercase in database.

DB - Mongodb,

Framework - Laravel 5.5

jenssegers/laravel-mongodb is using to connect laravel and mongodb

RegisterController.php

protected function validator(array $data)
{
  return Validator::make($data, [
     'firstName' => 'required|string|max:255',
     'lastName' => 'required|string|max:255',
     'email' => 'required|string|email|max:255|unique:user,usrEmail',
     'password' => 'required|string|min:6|confirmed',
  ]);
}

User.php

public function setusrEmailAttribute($value)
{
  $this->attributes['usrEmail'] = strtolower($value);
}

via Chebli Mohamed