Exchange's Spam Filtering Can't Be Turned Off?

While I'm a veteran spam fighter, I'm fairly new to Exchange Server administration. About two months ago I deployed Exchange Server 2003 for small web development firm I work part-time for.

My first goal was to disable all spam filtering Exchange shipped with. While spam does plague the company, we've also have had problems with false-positives. In one case it may have cost us some business.

I figured I'd start at ground zero, and then add what spam blocking was needed once I fully understood the options.

According to Microsoft, "spam confidence level" is a number between 0 and 9, indicating how confident Exchange is that a message is spam.

A level of 0 would suggest a message that's very unlikely to be spam, while a level of 9 would almost certainly be spam.

For the server-side ("Gateway") blocking option it seems possible to entirely disable message blocking. Presumably, if you choose "No Action" - it won't block anything no matter what the SCL is set to. Other options are to delete, archive or reject messages meeting the SCL level.

However, "threshold for moving UCE to a user's Junk E-mail folder" is only settable to a given SCL level. According to the description, all messages which are "spammy" enough to be an SCL 9 will get delivered to user's junk folder.

The SCL level is only settable from 0 to 9. There is no "disable" option.

This hit me as cause for alarm. Is it impossible to entirely turn off junk email filtering in Exchange server?

Even more concerning is that Exchange's spam detection is effectively a black box. Little information is available on what exact mechanisms it uses, and what criteria might be enough to qualify a message as having an SCL of 9.

Can you really not turn filtering completely off?

Some further research indicated this was actually not the case. It turns out someone at Microsoft made a boo-boo.

According to a knowledgebase article, The UI contains incorrect SCL rating text for the store threshold

The quick summary: The "Store Junk E-Mail Configuration" section of the UI incorrectly states "Move messages with an SCL rating greater than or equal to." The correct text should be the following (corrected text in bold): "Move messages with an SCL rating greater than."

So, setting the SCL threshold level to 9 in Exchange, is actually saying to process all messages with SCL's greater than 9. Since the SCL scale only goes up 9, no messages would ever meet the filtering criteria.

Setting the SCL level to 9 does effectively disable all spam protection in Exchange server.

I have since deployed a slightly novel anti-spam technique. I'm using Ipsec to block messages from countries we don't do business with.

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