Ed Brill

Even since Microsoft Exchange added clustering support, Exchange practitioners have always added a little asterisk when discussing its clustering capabilities.  Exchange 2000 clustering had significant resource consumption issues, with Active/Active clustering especially, and there was a time when it was simply not recommended.  That means Active/Passive clustering, also known as an expensive "spare tire".  A passive node sits in the cluster, waiting "just in case" of node failure.  In Exchange, this Active/Passive node couldn't do anything about failure at the shared disk, which left a single point of failure anyway.  

Apparently, the white flag is in full view at Microsoft TechEd Europe:

I'm in a Exchange clustering session now, the speaker just told the best practices on choosing active/active or active/passive scenarios for Exchange clustering.

I knew MS had active/active issues with Exchange clustering, I brought that in the open 4 years ago: http://veryold.bink.nu/exchange_2000.htm Big Smile [:D]

Now Microsft highly discourages active/active scenarious, saying if you have that running switch to a active/passive solution. In Exchange 12 active/active support will even not be available!Surprise [:O]

or, one supposes, you could switch to a Lotus Domino active/active solution, running Domino Access for MS-Outlook.  Another case where Lotus Domino improves your Outlook.....
Link: Bink.nu: TechEd Update: No Active/Active Fail-over cluster support in Exchange 12 > (via vowe.net)