Remember, I'm comparing to UNIX solutions. I've always had a few questions, like why can't I have all three nodes in a three-node cluster active? I'm forced to have one idle. What a waste. And do you realize it uses the JET Blue (MS Access) database engine? Ick!
If Microsoft wants to impress me, allow a full Exchange cluster, and have it use an SQL Server cluster as the back-end. Then both the data and the application will be in full failover.
This shows how little you know about Exchange and SQL. using SQL for a bunch of small messages would yield much lower performance. JET Blue isn't exactly Access, but it is close enough for comparisons. Having said that what makes Access a bad database is a strength for messaging (once modified).
I understand Microsoft looked at using SQL for exchange and it just couldn't pump the messages out quick enough. Completely different technology. Just because SQL makes a great database doesn't make it a great email server.