Customization
Mail filters have varying degrees of configurability. Sometimes they make decisions based on matching a regular expression. Other times, keywords in the message body are used, or perhaps the email address of the sender of the message. Some more advanced filters, particularly anti-spam filters, use statistical document classification techniques such as the naive Bayes classifier. Image filtering can also be used that use complex image analysis algorithms to detect skin-tones and specific body shapes normally associated with adult-images (pornographic images).
Read more about this topic: Email Filtering