Your address can be randomly spammed, it can be grabbed from someone's address book, and so on. (AOL was notorious for having their address books compromised.)
For business use, there really is no perfect filtering, other than manually setting up filters, and reviewing your spam folder.
If you need the easiest solution, gmail likely has the best filters (you can wash your mail through them via forwarding if you do not normally use their system). I used to use mailwasher, and it is a decent product. Caveat, gmail, and other online filters, do read your messages, and use the content to make money (serve ads).