The most common reason that a message isn't held as spam is, spammers are constantly coming up with new and inventive ways to evade spam filters. But, there are a couple of things you can check for in the headers.
1. Was the message sent to your Pobox address? If it doesn't include an X-Pobox-Delivery-Id: or X-Icg-Account-Id: header, then we didn't handle it. You'll need to talk to your ISP about why the message wasn't caught.
2. Was the message whitelisted? Look for an X-Pobox-Pass: header, to see if the message's sender has somehow made it onto your Trusted Sender list.
Beyond that, we recommend checking your Spam settings. Our recommended level of spam protection, if your account has been active for more than two weeks (and you've been actively reviewing your Spam section) is Aggressive. Also, consider turning on by-country blacklists. If you don't have any correspondents who live on a certain continent, you could take a bite out of your spam by blocking all mail from that contintent.