Ray, thanks for the tip.
1. Create a filter (you can do this while reading the message in question: click the down-arrow next to the Reply link, and select 'Filter messages like this' from the menu).
2. Enter the information of the messages that are being improperly labeled as spam (e.g. the sender's email address).
3. Click Next Step.
4. Select the checkbox next to 'Never send it to Spam.
Or: Put the senders address (email@example.com) in your contacts list.