Not to detract from Richards offer but there are two other options available.
Wash your current email through gmail or move to hosted exchange.
After years dealing, rather effectively, with a clients spam issue I finally managed to get them on hosted exchange.
This is a
PR firm so their email addresses are everywhere. We filtered 1.5M pieces of mail in one year and managed to filter 70% of the spam before hitting false positives.
Filtering is not easy. You need a combination of whitelisting, specific content based filters, subscribing to RBLs ( which are their own problem), configuring your servers to reject mail with improper headers or no PTR, DKiM, which a legitimate ending server will have set up properly but again not all the time.
Creating specific content filters is never ending. At some point it's near impossible to create them until after the spam arrives because you need to see what you are filtering for. Spammers know this and send in waves, you get the same crap multiple times simulataneously. They don't put enough in the email to allow for effective filters. You can set a filter for Invoice but if it's a business account that's going to cause false positives. And the next day it starts all over again with a new schema.
Gmail and hosted exchange work due to shear volume of clients. When you have millions of people you are handling mail for you are seeing a **** ton of spam that makes it easier to react to in real time resulting in the majority of your clients never seeing it.
Hosted exchange is a sweet deal. $12.50 a month gets you 99.9% spam free inbox plus licensing for the full office suite, word, excel, PowerPoint, outlook, on 5 concurrent devices Windows or Mac. You also get it on 5 concurrent mobile devices iOS, android, windows mobile. Plus a full terabyte of One Drive storage. There more benefits but honestly that is a massively good deal anyway you slice it.
And if it's for business, it's a write off.