If you find a message wrongly classified as spam, you can unmark the message. Just select the message, and click the Not Spam button that appears at the top and bottom of your current view. Unmarking a message will automatically move it to your inbox.
If you find that some senders' messages are consistently being mislabeled as spam, you can prevent this by:
Adding their email addresses to your Contacts list. We will deliver messages from members of your Contacts list to your inbox, unless we know with high confidence that they are spam.
Some messages sent from contacts which are very clearly spam can be sent directly to your Spam label. More importantly, in some cases messages from contacts will not be sent to Spam but will be marked with a red warning banner if the content is suspicious - for example, your friend's or contact's account has been compromised and used to send phishing messages.