Piwik\Plugins\UsersManager\Model::getUserByEmail PHP Method

    public function getUserByEmail($userEmail)
    {
        $db = $this->getDb();
        return $db->fetchRow("SELECT * FROM " . $this->table . " WHERE email = ?", $userEmail);
    }

 /**
  * Returns the first login name of an existing user that has the given email address. If no user can be found for
  * this user an error will be returned.
  *
  * @param string $userEmail
  * @return bool true if the user is known
  */
 public function getUserLoginFromUserEmail($userEmail)
 {
     Piwik::checkUserIsNotAnonymous();
     Piwik::checkUserHasSomeAdminAccess();
     $this->checkUserEmailExists($userEmail);
     $user = $this->model->getUserByEmail($userEmail);
     // any user with some admin access is allowed to find any user by email, no need to filter by access here
     return $user['login'];
 }