If a message you’ve sent never arrived at its destination,you should
first check your Sent Mail. Messages that you send to a mailing list,
or to an email address that’s automatically forwarding your mail to
Gmail,will only appear in your Sent Mail.

为什么gmail会这样做？这似乎是答案：Best practices for forwarding mail to Gmail

Authentication

• Do not modify,remove or shuffle message headers or modify the body of the message. DKIM or DomainKeys signatures will break if the body
  is altered,and all authentication information will be lost.
• Common DKIM breakages include modifying the mime boundaries,anti-virus or anti-spam programs modifying the body of the message,
  expanding the message recipient using LDAP,and re-encoding the
  message.
• Several highly-phished domains have asked Gmail to enforce strict authentication checks on their mail. These domains include eBay,
  Paypal,and Google. If your mail server modifies the forwarded
  messages from these domains,these messages will be marked as
  phishing.
• One symptom of this problem is that a messages from these domains forwarded through your mail server to Gmail will have an Authentication-Results header that show that DKIM did not pass,while messages sent directly to Gmail will have a passing signature.