Bob's right. Check your SMTP logs. You should be able to see every sender that hits your outside edge.

Alternatively, select Mail Security | SMTP | SPAM. Is your "Reject at SMTP time" on? If you turn it off it will let e-mail come further down the pipe.

What settings do you have in the blacklist section?

At the bottom of the page, which of the advanced settings are you using?

Yes, the latest SPAM filters are pretty tight right off the top, but I think most really like them. I do.