Mailwasher Pro is used & recommended by some eM Client users such as @skybat & @steveshank . See their posts below from the following Thread on programs for Spam Filtering.
Note:- If you do a “POP account” and use Mailwasher or any other external Spam filtering program, “disable the following option” via “Menu / Settings / General” under Synchronisation.
(eM Client V10 to disable POP mail checking on startup)