In my opinion, spam detection and filtering should not be done on the client side, - only on the server side, as the mail enters the server, before it is distributed to the user mailbox.
This way you avoid conflict between different clients, etc. (Imagine: Thunderbird, Aquamail, etc. are trying to figure out what is and what is not a spam, and at the same time... That's prone to race conditions, confusion, in short a recipe for a mess.)
... and that prevents bloating of the mail client.