我们使用Mailgun发送电子邮件，最近我注意到每天有相当多的电子邮件被Gmail拒绝。以下是我们收到的一种类型的消息：
我们做错了什么？
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规范是一份庞大的文件，所以我没有从头到尾地阅读它，但从网上的资源来看，我们的电子邮件没有落入任何会触发Gmail此类响应的常见陷阱。
这里是一个示例电子邮件头：
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？