How do I get MS Exchange to allow 3rd party pop3 redirection?

Hi,

I recently purchased the iphone and find it unusable without “direct push” email. Unfortunately, Apple only enabled Yahoo to provide this service for now. While I can access my corporate exchange server with a native pop3 client running from my PC, it will not allow Yahoo or MSN to login and redirect my emails. I need to do this so Yahoo will actually allow me to send emails using my corporate email address. When Yahoo or MSN attempts to log into my Exchange server, the connection is refused after a short period of time. Please help.

Thanks,

Jules