Mailwasher is only client-side filtering in a way - it downloads the headers rather than the whole mail, allows you to select/preselect what you don't want and then deletes them all from the server long before they hit your machine.
Yes, but
proper server-side filtering doesn't even store or parse the majority of spam emails. Over 80% of spam can be defeated at the initial SMTP transaction stage (SMTP HELO)... the remaining 19.99998% can then be defeated through analysis on the server.
Client-side filtering of any sort is a waste of time and computing resources.... its already too late.
Keep on prodding your email provider to improve their anti-spam.