In other words I'd like to be able to filter client mail based on subject so that it redirect out of their mailbox to postmaster@mydomain.com without them ever knowing it was there. So if the subject was [Suspected SPAM] could i have an exchange rule grab any mail in their mailbox with that subject and redirect it to postmaster all in the back-end so the outlook client thinks it never even received mail with such a subject?

Is this what you are looking for?http://office.microsoft.com/en-us/outlook/HA011502011033.aspx

That depends on your Exchange Server version. If you are using Exchange 2007, you can have this choice. If you are using Exchange 2003/2000, you may think about this via Outlook rule.