As plenty of others here have mentioned, if you're willing to dive in and maintain it, the tools are available. However, it's probably not going to be worth it to you if you want to avoid the sysadmin side of things.
That said, I'd take a look at putting Zarafa (www.zarafa.com) (Groupware, like Zimbra, but native linux code with some cool features like free Exchange Activesync support) on top of a postfix + spamassassin setup to get a really robust set of open-source services going. Add in Maia Mailguard (http://www.maiamailguard.com/) to add automation & central administration to your spam filtering, and you'll likely be pretty happy with your setup (if you're willing to spend the time to put it all together.
