Google recently introduced a feature, now available in BergApps, that helps you learn why an incoming email message was labeled as Spam. While reading a message in your Spam folder, you'll see a new info box at the top of the message indicating why Google's automated filters flagged the message.
For more information on this feature, visit this post from the Official Gmail Blog.
Don't forget that you can also help train Google's spam filters by unmarking legitimate messages that were wrongly flagged as well as by reporting unflagged spam messages.;
To learn more about BergApps, visit our BergApps Support Center.