[baudehlo]

Oh you absolutely can, but there are a mix of problems some of which makes spam filtering better on the server (knowing bad sources of email, like infected desktop machines), some of which makes filtering better on the client (like knowing your users' preferences for what they consider spam).

There's a huge long history of very smart people (not even including myself in that list - I've worked with some people MUCH smarter than myself on this) working on this problem, including the very creator of this site, pg himself, and even he doesn't consider this a solved problem. People are still paid well to combat spam every day on behalf of demanding customers (and I'm grateful I'm not one of them any more) and it will always require new techniques and new input. Hopefully you will contribute too.