Why is it so difficult to get spam control right?

Inbound email messages are sent through several security and hygiene processes before being rejected or accepted. One of these processes is a rigorous anti-spam screening, which assigns a “spam score” to each message based on a number of message attributes. If high enough, the score causes that message to be delivered to your Spam folder instead of your Inbox. Since the amount and kinds of spam vary over time, the scoring system's effectiveness also varies somewhat and must sometimes be re-calibrated to the latest spam. In addition, a message may be considered spam by one person but not by another. For these reasons and others, there is no perfect scoring system that identifies spam correctly for everyone.