请考虑以下电子邮件标题，并将其发送回原始发件人。 多部分退回消息还包含原始发件人和预期收件人之间的对应关系的抄本（明文+ HTML），我将其剥离。
很长一段时间以来，我觉得我对（E）SMTP相当熟悉，现在看起来不是这样了。 假设这不是假的反弹，我只能猜测已经指定了不同的信封收件人（发件人已被黑客入侵？），或者收件人的MTA中包含一些密件抄送地图或别名function。
以下实体已被其各自的占位符replace：
%original sender% – 实际将退回邮件转发到%intended-recipient% %intended recipient% – 担心自己安全性的人，因为％original-sender％转发了她此退回邮件 %totally unrelated%@gmail.com – 某人的电子邮件地址，无论是％original-sender％还是％intended-recipient％都与之对应。
来自：邮件传递系统[mailto：Mailer-Daemon @]
发送date：2013年10月17日，星期四下午1:22
要： ％原始发件人％
主题：邮件投递失败：将邮件退回给发件人
此邮件是由邮件传送软件自动创build的。
您发送的消息无法传递给其中的一个或多个
收件人。 这是一个永久性错误。 以下地址失败：
**%totally unrelated%** @gmail.com SMTP error from remote mail server after end of data: host gmail-smtp-in.l.google.com [2a00:1450:4001:c02::1b]: 550-5.7.1 [ **%currently unreachable IPv6 address%** 16] Our system has detected 550-5.7.1 that this message does not meet IPv6 sending guidelines regarding PTR 550-5.7.1 records and authentication. Please review 550-5.7.1 https://support.google.com/mail/?p=ipv6_authentication_error for more 550 5.7.1 information. u47si3796594eel.284 - gsmtp ------ This is a copy of the message, including all the headers. ------ Return-path: <**%original sender%**> Received: from **%MX of intended recipient%** ([ **%IP address of MX of intended recipient%** ]) by **%another MTA in the network of recipient's ISP%** with esmtp (Exim 4.63 #1 (Debian)) id 1VWleU-0006QW-Pl from < **%original sender%** > for < **%totally unrelated%** @gmail.com>; Thu, 17 Oct 2013 13:22:02 +0200 Received: from **%MTA of sender's ISP%**([**%IP of MTA of sender's ISP%**]) by **%MX of intended recipient%** with esmtp (Exim 4.69 #1 (Debian)) id 1VWleU-0004oq-JG for <**%intended recipient%**>; Thu, 17 Oct 2013 13:22:02 +0200 Received: from **%sender's hostname%** (**%sender's PTR%**[ **%sender's IP%** ]) by **%MTA of sender's ISP%**(Postfix) with ESMTPA id 3d0nyG0hwJz1741X for <**%intended recipient%**>; Thu, 17 Oct 2013 13:21:41 +0200 (CEST) From: **%sender's name encoded%** <**%original sender%**> To: **%intended recipient's name encoded%** <**%intended recipient%**> References: <000c01cecb17$d9321ef0$8b965cd0$@tlh> In-Reply-To: <000c01cecb17$d9321ef0$8b965cd0$@tlh> Subject: **%subject line encoded%** Date: Thu, 17 Oct 2013 13:21:41 +0200 Message-ID: <00b701cecb2b$0dfd9d40$29f8d7c0$@**%sender's domain%**> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_00B8_01CECB3B.D1866D40" X-Mailer: Microsoft Outlook 14.0 Thread-Index: AQGITggPsq7UFiUmFwcnkdC2rH3HB5qFuXTQ Content-Language: tlh This is a multipart message in MIME format.
最终用户通过MTA转发他们的电子邮件（如/ etc / aliases）。