code: private function validateEmail($value)
      {
          $isValid = filter_var($value, FILTER_VALIDATE_EMAIL);
          if ($isValid === false) {
              $this->throwInvalidArgumentMustBe("valid email");
          }
      }
docs: "I found some addresses that FILTER_VALIDATE_EMAIL rejects, but RFC5321 permits"
  http://php.net/manual/en/function.filter-var.php#112492
  
  @param $value