Accuracy and False Positives

Issue with filtering to date: not just "how much spam does it catch?" but "how many legitimate mails get caught, too?"

Many systems do not pay attention to this problem

Some DNS blacklists use "false positives" as a weapon against service providers selling to spammers!

False Positives (non-spam marked as spam) are much worse than spam getting through; much more inconvenient to user

No classifier is perfect; even humans get it wrong