Nearly all of Pobox's spam filters check the reputation of the computer that sends us mail. And, normally, that's very, very accurate. However, if you forward your mail to your Pobox address from another email provider, it doesn't work at all. Why?
Let's use Go Daddy as an example (though the same is true of any ISP, webmail or legitimate email provider.) If you set up your Go Daddy account to forward your mail to Pobox, we check the reputation of the Go Daddy computer that sends us the mail. Except in very unusual circumstances, the reputation of that computer should always be good, so mail coming from that computer will never get caught as spam.
There are several things you can do, though, so that we'll catch more spam.
1. Give people your Pobox address. Mail that goes straight to Pobox will always be checked directly.
2. If you're using a domain, set up the domain at Pobox. You can move your email to us, without affecting your website.
3. Tell Customer Service where you're forwarding your mail from. Tell us you have an "in-forwarding" address, and give us the address. We'll look up the names of the computers where that mail comes from, and set up a new filter for you. The new filter will look one step up the chain, and check the reputation of the computer that sent the mail to your "in-forwarding" address, instead of the reputation of your "in-forwarding" address.
If you don't forward your mail to us from another domain, but changes to the spam slider don't seem to affect the amount of spam you're getting, the next most common reason is that spam is being sent directly to your forwarding address. Pobox Customer Service can help you try to figure out what's going on, though. Get in touch with us, and we'll talk about what we can do.