I've been running hMailServer at home for at least 6 months. It's used by only one person (me!) to automatically pull my email from my ISP and then serve it back via IMAP -- thus effectively acting as my central email archive. It's configured to store its metadata in a SQL Server 2005 database (free copy from Microsoft), and I haven't had any problems with it. The first version I used was slow, but the version I'm running now is much better. And I believe there is at least one newer version available that I haven't yet grabbed.