Has somebody experience in running an large Exchange/W2K3 2003 environment with 20.000 or more mailboxes on ESX 3? At this moment we have 5 physical servers with 4.000 mailboxes each .
I think you have to look at the whole picture. If you can manage more exchange servers, virtualization brings a lot to the table, especially as new features are rolled out. DRS and HA are great features that are available now, Continuous Availabiltity (coming soon) will make the system nearly bulletproof without having to try to configure Microsoft Clustering Services.
With an implementation that large, there would be several factors to consider. Are you planning a 1:1 move to take advantage of DRS/HA or putting multiple boxes on a single host to also consolidate? Obviously IOPS would be an issue, but how are you storing the data now - fiber channel SAN? There should be no issue with procs or RAM as ESX3 should be able to address more than your current servers. Are you considering a move to 2007? That would drive down the IOPS requirement and also give you 64-bit memory allocation.
Just keep in mind that Microsoft does not support Exchange 2003 being virtualized on any platform other than Microsoft Virtual Server 2005 R2. They do not even support it on Hyper-V. Exchange 2007, however, is supported on both Hyper-V and ESX.
Microsoft's recommendations and support policy for virtualized Exchange servers can be found here:
