Lately we got a lot of spam because we bought the domain timr.com for our product and it seems like the previous owner has not been very carefully with his email address. Spam is annoying but it is even more annoying if you get it pushed to your BlackBerry. So I tried to find the best solution for adding a spam filter to our mail server.
We are using a Microsoft Exchange Server 2007 on an Windows Small Business Server 2003 because in our opinion this is one of the best solution available in combination with the BlackBerry Enterprise Server. I have a lot of experience with Linux mail servers because I run my personal mail on a Debian system with IMAP, spamassassin, greylisting and maildrop for years now. This combination removes nearly every spam mail from my email so I was looking for combining it with our company mail system.
Though there are attempts to install spamassassin on Windows and to combine it with Exchange we were not quite happy with that solution. I was already thinking about subscribing to some service like Spam Stops Here. Using these services means changing the MX DNS entry of your domain to send all emails through their server which forwards them afterwards to your mail server.
This inspired me to create our own dedicated mail filtering server. I set up a Ubuntu 9.04 machine inside a VMWare Virtual Server instance on our Windows machine and configured it like my personal mail server without the IMAP server. Instead I used the how to from knownplace to forward the filtered mail to our Exchange server. After changing the MX entry of our domains to point to the new Linux mail server the amount of spam mails dropped drastically.