From the testing I have done, when using keywords the entire email is searched excluding attachments. Filtering by subject only is too easy to bypass with false or empty subject lines. This is the main problem with SPAM in the first place. From the hundreds of thousands of emails I have filtered, very few were filtered by a keyword in the subject line. The smart SPAMMERS have known for a long time that if they were to put the true topic in the subject line, most people would just trash it with out reading it. Just like junk postal mail.

Now on to "censorship" This is a touchy subject but must be addressed. Yes in a sense filtering any email could be considered censorship which is the number one argument that Pro-SPAM pundits use to justify what their right to send SPAM. Here is the problem, SPAM - Bulkemail is un-asked for in most cases and costs ISP's and email users both time and money. Due to the abuse of the openness of email standards, bulkmailers have abused it to the point where email is rapidly becoming a useless tool.

Until the next version of SPAMFilter is released that supports the web interface which will allow the email user the ability to screen the email before it is downloaded there are only a few options. What I have offered my ISP clients is if they done want filtered email then they can have it unfiltered. Very few of them want it unfiltered. Most of them are so pissed off because of the massive amount of garbage, they would rather get no email then have to delete 100's of penis enlargment offers, Viagra, teen sex, porn, mortagage ads and the many other garbage emails that they recieve.

And the idea that the government should pass a anti-spam law that would be based on an opt-out only system is a joke. It should be an opt-in system with the ability to really opt-out anytime there after. Then people would have no reason to complain about getting email advertisments since they requested it in the first place. But the chances of this happening are very slim.

As the owner of a national ISP, I see things from the view point of one who has to pay for equipment, bandwidth, and technical support. Of all the problems I have to deal with, email is by far the most costly and as most ISP's we don't charge extra for email, it is just another part of the whole service.

Just to clarify something. Most of the SPAM that is blocked ends up being rejected for the following reasons.
1) No reverse DNS
2) Blacklisted in 2 or more RBL
3) Keywords, either in the subject of the email or in the body

I agree that adding a seperate subject line only filter would help filter out certain spam.

There are some mass-spam applications that generate bulk email and create a subject line that has a generic subject, a series of spaces and finally tracking text (usually garbled letters or numbers).  One way to filter against these type of spam is to filter out any subject line that has maybe seven or more spaces in a subject line.  Even if the content of the email is obscured using HTML tricks that the regular keyword filters would not be able to read, this would catch any email generated by one of these mass-email apps.

It's a good idea and is worth considering as an additional filter.