我正在尝试通过phpmailer来"回复"电子邮件，但不是"发送新电子邮件"，但是我甚至在内置上和参考已添加到电子邮件标题，这意味着它仍然是一封新电子邮件，原始电子邮件从未回复。感谢您的帮助。
我的代码：
if($this->MessageID != '') {
$result .= $this->HeaderLine('Message-ID',$this->MessageID);
} else {
$result .= sprintf("Message-ID: <%s@%s>%s", $uniq_id, $this->ServerHostname(), $this->LE);
}
if($this->References != ''){
$result .= $this->HeaderLine('References',$this->References);
}
if($this->InReplyTo != ''){
$result .= $this->HeaderLine('In-Reply-To',$this->InReplyTo);
}
$result .= $this->HeaderLine('X-Priority', $this->Priority);
$result .= $this->HeaderLine('X-Mailer', 'PHPMailer '.$this->Version.' (phpmailer.sourceforge.net)');
通过phpmailer发送的电子邮件标头（我失败，这意味着仍然是新的电子邮件，但没有回复）
Date: Sat, 3 Dec 2016 17:01:21 +0800
To: Eric <erichu@test.com>
From: support <support@test.com>
Subject: Re:testemail12031604
Message-ID: <tencent_0634F549045841EE1B94753E@test.com>
References: <tencent_205C6E0F42D57AD16CFF6B9C@test.com>
In-Reply-To: <tencent_205C6E0F42D57AD16CFF6B9C@test.com>
X-Priority: 3
X-Mailer: PHPMailer 5.1 (phpmailer.sourceforge.net)
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="b1_39519ee60f388e1a9b9a9ec312ae2071"
这是成功回复电子邮件的标题，该回复在Web中
From: "=?utf-8?B?RWxhaW5l?=" <support@test.com>
To: "=?utf-8?B?RXJpYw==?=" <erichu@test.com>
Subject: Re:hubreplytestemail12031604
Mime-Version: 1.0
Content-Type: multipart/alternative;
boundary="----=_NextPart_5842871D_0A6F1F10_2B4284B6"
Content-Transfer-Encoding: 8Bit
Date: Sat, 3 Dec 2016 16:49:33 +0800
X-Priority: 3
Message-ID: <tencent_477F7F80668628EB4A87C531@test.com>
X-QQ-MIME: TCMime 1.0 by Tencent
X-Mailer: QQMail 2.x
X-QQ-Mailer: QQMail 2.x
References: <tencent_205C6E0F42D57AD16CFF6B9C@test.com>
In-Reply-To: <tencent_205C6E0F42D57AD16CFF6B9C@test.com>
X-QQ-ReplyHash: 2325734638
这看起来就像您在使用phpmailer的旧版本；多年来，它一直在SourceForge上保持不变，2010年发布了5.1.0！获取最新信息。您为什么还要查看phpmailer的内部？您应该只打电话给addCustomHeader()添加所需的标题：
$mail->addCustomHeader('References', '<tencent_205C6E0F42D57AD16CFF6B9C@test.com>');
$mail->addCustomHeader('In-Reply-To', '<tencent_205C6E0F42D57AD16CFF6B9C@test.com>');