Nice post blogged in MSExchangeTeam.com about ESE and SQL.
As most of you know, Exchange servers utilize the Extensible Storage Engine (ESE) database. As you have told us loud and clear, Exchange rocks from a performance, scalability, and availability standpoint on ESE and we've worked hard to make it so.
However, for some time (it has been a topic of discussion since the development days of Exchange 2003) there have been questions and rumors around replacing the ESE database with SQL Server.
- Did the team consider using SQLServer for the next version of Exchange? - Absolutely!
- Did it work and perform well? - Yes! Some very smart engineers did amazing work, and we had mailboxes up and running using a SQLServer database.
- Did we ultimately decide to replace the ESE database with SQL Server for the next version of Exchange Server - No.
Read more at source: http://msexchangeteam.com/archive/2009/07/16/451784.aspx