How do I prevent messages from specific senders from being tagged as spam?

There are a couple of ways of handling messages that are mislabeled as spam.  If individual messages are identified as spam, you can unmark the message by selecting the message and clicking the Not Spam button that appears at the top and bottom of your current view.  The email will then be moved into your inbox.

If messages that you receive from specific persons or groups outside our domain continually get tagged as spam, you can prevent this from happening by adding their email addresses to your Contacts list.  Gmail will then deliver the message to your inbox unless Google knows, with high confidence, that they are spam. Additional information on Spam can be found at the Google Help site.  Information on Creating Contacts can be found here.