Have you ever peered into your spam folder on Gmail and wondered how and why those 600 messages were marked as spam? Gmail now provides you with an explanation for how those messages ended up there so you can better understand them.
When you select your spam folder, click on any message, and at the top of the message you'll see an explanation for why the message was marked with spam. These range from the boring old, "It's similar to messages that were detected by our spam filters", to the more exciting, "It might contain a virus or malicious link." Next to each message is a Learn More link that will lead you to Gmail's series on spam that tell you if you need to take action to stop the spam or if you can let it be. It can be helpful if your spam folder is filled with phishing scams, but it's also handy for those one or two messages from friends that seem to end up in your spam folder for no reason. The feature should be live in your Gmail account right now.
Learn why a message ended up in your spam folder [Official Gmail Blog]