Friends List:
Effectiveness: High
Advantages: Significantly reduces false positives. (False positives are good emails that are improperly categorized as spam.)
How does it work: Any email message coming from a user in your Friends list is allowed to completely bypass all other filters. It is automatically allowed through and into your Inbox.
Limitations: Friends may still occasionally send chain letters and other messages you may not want to read.
Recommendations: The Friends list should be enabled for almost every user. There is virtually no downside and you can rest easy that once a user is on your Friends list any email from that user will not be accidentally misclassified as spam. We highly recommend leaving this option turned on.
