If you look at the e-mail header in the e-mail source, you will see a line that looks similar to the following:
X-Spam-Status: No, hits=1.7 required=2.0
In the above example, 1.7 represents the SPAM score. 2.0 represents your “Tagging Threshold” setting in the control panel. For this message to have “[SPAM]” in the subject line, it would have to have a score above 2.0, and in this case it does not.