As a part of its effort to be more transparent about its practices, Google has been sending out a notification to webmasters alerting them when unnatural links are detected. The notice itself serves as an FYI and doesn’t necessarily mean that a penalty has occurred (yet). The only way to really know if you have a penalty is if you notice that the warning coincides with a significant dip in traffic. If you’ve been handed an unnatural link penalty, it’s important to do everything that you can in order to remedy the situation.