public function checkIsValidEmailOrEmpty(
        $fieldName,
        $canUseFlexforms,
        $sheet,
        $unused,
        $explanation
    ) {
        $value = $this->objectToCheck->getConfValueString($fieldName, $sheet);
        if ($value === '') {
            return;
        }

        if (!GeneralUtility::validEmail($value)) {
            $message = 'The e-mail address in <strong>' . $this->getTSSetupPath()
                . $fieldName . '</strong> is set to <strong>' . $value . '</strong> '
                . 'which is not valid. E-mails might not be received as long as '
                . 'this address is invalid.<br />';
            $this->setErrorMessageAndRequestCorrection(
                $fieldName,
                $canUseFlexforms,
                $message . $explanation
            );
        }
    }