我们使用Mailgun发送电子邮件，最近我注意到每天有不少电子邮件被 Gmail 拒绝。
这是我们收到的消息类型：
550
5.7.1 [184.173.153.6 11] Our system has detected that this message is
5.7.1 not RFC 2822 compliant. To reduce the amount of spam sent to Gmail,
5.7.1 this message has been blocked. Please review
5.7.1 RFC 2822 specifications for more information. f15si23385851vdu.1 - gsmtp
RFC 2822规范是一个庞大的文档，所以我没有从头到尾阅读它，但是通过查看网络上的资源，我们的电子邮件不会落入任何会触发 Gmail 此类响应的常见陷阱。
这是一个示例电子邮件标题：
Received: by luna.mailgun.net with HTTP; Mon, 29 Jun 2015 21:06:59 +0000
Message-Id: <20150629210659.18668.39318@(domain)>
X-Mailgun-Variables: {"variation": "original", "campaign_code":
"(customValue)"}
Reply-To: (name) <(email)>
X-Mailgun-Track: false
X-Mailgun-Tag: (customTag)
To: (email)
From: (name) <(email)>
Subject: (subject)
Mime-Version: 1.0
Content-Type: multipart/alternative; boundary="12f0bd630f2145a3afcd98b621a3b1f2"
--12f0bd630f2145a3afcd98b621a3b1f2
Content-Type: text/plain; charset="ascii"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
(text content)
--12f0bd630f2145a3afcd98b621a3b1f2
Content-Type: text/html; charset="ascii"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>(title)</title>
<style type="text/css">
(css)
</style>
</head><body style="(css)" >
(content)
</body>
</html>
--12f0bd630f2145a3afcd98b621a3b1f2--
我们做错了什么？