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. 

I did some searching and i found a few different solutions depending on what platform your server runs and what smtp server you are running. Here is a list of most of the implementations.  

So here is the long and short of it; i started with almost zero filtering and zero spam, 5 years later i still had almost zero filtering but was getting 50-100 spam messages a day.  I installed a grey listing product and now i have zero spam.  in fact i have been running greylisting for about two months and i have had two spam messages make it past the filtering! I also have over 90% of my incoming mail is filtered or marked as spam.  so for each valid mail message that i get in my inbox, i have already dropped 9-10 spam messages.  

damn greylisting is slick!
but it does have a few faults the most noticeable one being that before a domain or smtp server gets added to the white list all mail from that domain can take a long time to show up in your inbox.  This is just the nature of the beast and this is how greylisting works. After the white listing of the domain then everything should from that point on be just as fast as normal.  Another issue is that some smtp servers will simply take that 450 error and consider it as a serious failure and not try to resend the mail. **note: i have not noticed this YET. One more little issue that i have just learned to deal with is while setting up a new account on the web somewhere a lot of sites require you to respond to an email to complete the account setup process, and the odds are that you have prob never had an email from this domain before and because of that you don’t have them on your white list, this delays the whole account setup process and you have to wait.  I have found that on average most mail will get me my mail in about 10-15 mins, and i can live with that.