If it is a server level spam filter -- that is, if the spam is filtered before even going into your inbox -- then it shouldn't even be going to your BlackBerry. If that is the setup, have you looked to see if the spam is in fact not being filtered? And have you tested to confirm that ligit email is not coming to the BlackBerry, and that it is going to the inbox and not to the BlackBerry?

For what it's worth, weird email problems are often fixed by deleting the address from BIS and then adding it back. A typical way to do it would be to delete the address in BIS, delete the service books for that address on the BlackBerry, wait a while (a couple of hours to half a day), then add the address back to BIS and resend service books.