It sounds like you have no SPAM filters at all, until MS, with that setup. Have you tried tweaking the SPAM settings there. (Sorry, I don't know how to do that) As you've got a Mac, why not try using its Mail application. Apple's filters may not be perfect but they do reduce the volume considerably.
When you set up Mail, you don't have to turn off your MS option, you will get the messages on both and can compare.