Source : http://technet.microsoft.com/en-us/library/dd346700.aspx
Memory configurations for Exchange 2010 servers based on installed server roles
| Exchange 2010 server role | Minimum supported | Recommended maximum |
|---|---|---|
| Edge Transport | 4 GB | 1 GB per core (4 GB minimum) |
| Hub Transport | 4 GB | 1 GB per core (4 GB minimum) |
| Client Access | 4 GB | 2 GB per core (8 GB minimum) |
| Unified Messaging | 4 GB | 2 GB per core (4 GB minimum) |
| Mailbox | 4 GB | 4 GB plus 3-30 MB additional memory per mailbox: The total required memory is based on the user profile and database cache size. For more information about how to determine the total required memory, see Understanding the Mailbox Database Cache. |
| Client Access/Hub Transport combined role (Client Access and Hub Transport server roles running on the same physical server) | 4 GB | 2 GB per core (8 GB minimum) |
| Multiple roles (combinations of Hub Transport, Client Access, and Mailbox server roles) | 8 GB | 4 GB plus 3-30 MB additional memory per mailbox: The total required memory is based on the user profile and database cache size. For more information about how to determine the total required memory, see Understanding the Mailbox Database Cache. |
Edge Transport and Hub Transport Server Roles The Edge Transport and Hub Transport server roles don't require substantial quantities of memory to perform well in optimal conditions. Generally, 1 GB of RAM per processor core (4 GB minimum total) is sufficient to handle all but the most demanding loads. Most deployments will be optimally configured with the recommended memory configuration of 1 GB per processor core (4 GB minimum total).
Client Access Server Role In general, memory utilization on Client Access servers has a linear relationship with the number of client connections and the transaction rate. Based on the current recommendations of 2 GB per core processor and memory configurations, a Client Access server will be balanced in terms of memory and processor utilization, and it will become processor-bound at approximately the same time it becomes memory- bound.
These recommendations are based on the Exchange 2010 feature, RPC Client Access. This feature requires a larger memory and processor configuration to manage the increased loads placed on the Client Access server role.
