Gmail has implemented new and strict policies on spam messages and they now include messages that explain why they are in your spam folder.

To get that information, log in to your gmail account, go to spam, and click on the message, it will list one or more of these spam reasons:
  • Why is this message in Spam? We’ve found that lots of messages from are spam.
  • Why is this message in Spam? It’s similar to messages that were detected by our spam filters.
  • Why is this message in Spam? It contains content that’s typically used in spam messages.
  • This message was likely forged and did not originate from your account.
  • This message may not have been sent by:
  • Be careful with this message. Similar messages were used to steal people’s personal information. Unless you trust the sender, don’t click links or reply with personal information.
  • Be careful with this message. Many people marked similar messages as spam.
  • Be careful with this message. It might contain a virus or a malicious link.
For more information, please refer to Google's documentation regarding this matter. A good troubleshooting step to take would be to send the same email to a different email service provider (such as Yahoo) and see how they treat the email message. The issue could end up being something specific to Gmail's spam filter and the content of your message that most other Email Providers do not consider a threat.