If you find a message that should be marked as spam, just move it to '[Gmail]/Spam.' This is just like clicking 'Report Spam' in the Gmail web interface and helps us to improve our spam filters.

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. Gmail 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.
• Creating a filter so the messages are never sent to Spam.