Spam consumed 40% of our e-mail traffic last year. Whilst there are two ways of managing the issue, server and client-side, it is best that we employ both. Personally, I have GMail's aided by my mail client's spam filter plug-in, which was set to be aggressive. :)
On the server side, spamd is one of the best known spam filters available in the open source arena today. So taking from the tutorial of Terrell Prude, Jr. on how to install and configure spamd on OpenBSD, he has this to say about the power of this nifty tool,
Spamd's greytrapping is phenomenal and a significant part of why I get that huge reduction. My spamd logs show large numbers of folks attempting to spam my single fake email address, and all of them get tarpitted and rejected.
So, free your users from spam and get this awesome spam filter up and running now!