The current rules functionality is too limited to filter most spam. The only fields we can match are:

• Contact email (useless because of spoofing and because spammers can change domains as many times as they want)
• Message subject (somewhat useful for spam written in Western alphabet; useless for spam written entirely in Chinese, which is what I'm buried with)
• Message text (see above)

Additional useful match fields would be:

• Destination address
• Message HTML (potentially useful if you can't match by message text, see below)
• Additional email header fields

Furthermore, the only matches are "includes" and "does not include". Useful spam filters have options such as "is exactly", "starts with", "ends with", etc.