Summary: Information about how emails are filtered for spam in Gmail, and what to do if a spam message finds it's way to your inbox, or a legitimate message is marked as spam.
As of November 2014, AESHS transitioned from Postini to Google Apps/Gmail spam filtering. This transition means that we now rely on Google to process and mark messages as spam, to keep your Gmail inbox free of unwanted messages.
As no spam filter is 100% perfect, sometimes spam will find it's way to your inbox. To remove spam from your inbox...
More information can be found in Google's Gmail Help Forum: https://support.google.com/mail/answer/6602
Even Google will sometimes mark legitimate emails (ham) as spam. If you find legitimate emails in your spam folder, you can follow this process, detailed by Google, to move those emails to your inbox...
If you find a message wrongly classified as spam, you can unmark the message. Just select the message, and click the button that appears at the top and bottom of your current view. Unmarking a message will automatically move it to your inbox.
More information can be found in Google's Gmail Help Forum: https://support.google.com/mail/answer/9008
If you go into your Spam folder, and open one of the messages marked as spam, usually you will see a warning label at the top of the message with a brief explanation about why that particular message was placed in Spam. For more information about those messages, and what they mean, Google's Gmail Help Forum has an explanation:
Google's Official Gmail Blog also has a nice explanation of why messages are marked as spam, and the warning labels you may see: