I have been running my own mail server for about 6 years or so and it took almost 5 years for me to start getting spam on most of my accounts.  The thing about spam is that it really never decreases, you continue to get more and more and more and they never take you off the list, no matter what you do.  I have tried a few tricks to attempt to get removed from a few lists but really nothing works.  For the last 6 months i almost feared my inbox and would notice that i checked it less and less each week, this is really not a viable option because i was getting ~100 spams a day.  I know that for most people that’s not really a huge number but i have had almost zero filtering on my inbox for over 5 years and all was great.  So after talking to one of my buddies, who also run a mail server as well, he told me about the concept of greylisting and how it works... i will not drive into the details of how it works but here is the info. 

More...