PHPMailer::createHeader(): string

2850    public function createHeader()2851    {2852        $result = '';2853 2854        $result .= $this->headerLine(2855            'Date',2856            self::sanitiseDate($this->MessageDate)2857        );2858 2859        //The To header is created automatically by mail(), so needs to be omitted here2860        if ('mail' !== $this->Mailer) {2861            if ($this->SingleTo) {2862                foreach ($this->to as $toaddr) {2863                    $this->SingleToArray[] = $this->addrFormat($toaddr);2864                }2865            } elseif (count($this->to) > 0) {2866                $result .= $this->addrAppend('To', $this->to);2867            } elseif (count($this->cc) === 0) {2868                $result .= $this->headerLine('To', 'undisclosed-recipients:;');2869            }2870        }2871        $result .= $this->addrAppend('From', [[trim($this->From), $this->FromName]]);2872 2873        //sendmail and mail() extract Cc from the header before sending2874        if (count($this->cc) > 0) {2875            $result .= $this->addrAppend('Cc', $this->cc);2876        }2877 2878        //sendmail and mail() extract Bcc from the header before sending2879        if (2880            (2881                'sendmail' === $this->Mailer || 'qmail' === $this->Mailer || 'mail' === $this->Mailer2882            )2883            && count($this->bcc) > 02884        ) {2885            $result .= $this->addrAppend('Bcc', $this->bcc);2886        }2887 2888        if (count($this->ReplyTo) > 0) {2889            $result .= $this->addrAppend('Reply-To', $this->ReplyTo);2890        }2891 2892        //mail() sets the subject itself2893        if ('mail' !== $this->Mailer) {2894            $result .= $this->headerLine('Subject', $this->encodeHeader($this->secureHeader($this->Subject)));2895        }2896 2897        //Only allow a custom message ID if it conforms to RFC 5322 section 3.6.42898        //https://www.rfc-editor.org/rfc/rfc5322#section-3.6.42899        if (2900            '' !== $this->MessageID &&2901            preg_match(2902                '/^<((([a-z\d!#$%&\'*+\/=?^_`{|}~-]+(\.[a-z\d!#$%&\'*+\/=?^_`{|}~-]+)*)' .2903                '|("(([\x01-\x08\x0B\x0C\x0E-\x1F\x7F]|[\x21\x23-\x5B\x5D-\x7E])' .2904                '|(\\[\x01-\x09\x0B\x0C\x0E-\x7F]))*"))@(([a-z\d!#$%&\'*+\/=?^_`{|}~-]+' .2905                '(\.[a-z\d!#$%&\'*+\/=?^_`{|}~-]+)*)|(\[(([\x01-\x08\x0B\x0C\x0E-\x1F\x7F]' .2906                '|[\x21-\x5A\x5E-\x7E])|(\\[\x01-\x09\x0B\x0C\x0E-\x7F]))*\])))>$/Di',2907                $this->MessageID2908            )2909        ) {2910            $this->lastMessageID = $this->MessageID;2911        } else {2912            $this->lastMessageID = sprintf('<%s@%s>', $this->uniqueid, $this->serverHostname());2913        }2914        $result .= $this->headerLine('Message-ID', $this->lastMessageID);2915        if (null !== $this->Priority) {2916            $result .= $this->headerLine('X-Priority', $this->Priority);2917        }2918        if ('' === $this->XMailer) {2919            //Empty string for default X-Mailer header2920            $result .= $this->headerLine(2921                'X-Mailer',2922                'PHPMailer ' . self::VERSION . ' (https://github.com/PHPMailer/PHPMailer)'2923            );2924        } elseif (is_string($this->XMailer) && trim($this->XMailer) !== '') {2925            //Some string2926            $result .= $this->headerLine('X-Mailer', $this->secureHeader(trim($this->XMailer)));2927        } //Other values result in no X-Mailer header2928 2929        if ('' !== $this->ConfirmReadingTo) {