private function addRecipientEmails(
        $emailType,
        $emails,
        $personalizationIndex = null,
        $personalization = null
    ) {
        $emailFunctionCall = "add" . $emailType;

        if (current($emails) instanceof EmailAddress) {
            foreach ($emails as $email) {
                $this->$emailFunctionCall(
                    $email,
                    $name = null,
                    $personalizationIndex,
                    $personalization
                );
            }
        } else {
            foreach ($emails as $email => $name) {
                $this->$emailFunctionCall(
                    $email,
                    $name,
                    $personalizationIndex,
                    $personalization
                );
            }
        }
    }