protected function addRecipient($mUserIdOrEmail, $bAppend, &$aArray)
    {
        if (!$bAppend) {
            $aArray = [];
        }

        if (strpos($mUserIdOrEmail, ',') !== false) {
            $mUserIdOrEmail = array_map('trim', explode(',', $mUserIdOrEmail));
        }

        if (is_array($mUserIdOrEmail)) {
            foreach ($mUserIdOrEmail as $sUserIdOrEmail) {
                $this->addRecipient($sUserIdOrEmail, true, $aArray);
            }
        } else {
            $this->validateEmail($mUserIdOrEmail);
            $aArray[] = $mUserIdOrEmail;
        }

        return $this;
    }