- 1GB per CPU core for Edge Transport and Hub Transport servers
- 2GB per core for Unified Messaging and Client Access servers
- For Mailbox servers, 4GB plus between 3MB and 30MB per mailbox (so somewhere between 7GB and 34GB for a 1,000-mailbox server)
- 2GB per core for servers that combine Hub Transport and Client Access servers
- For servers that combine the Mailbox role with other roles, a minimum of 8GB (4GB plus 3-30MB/mailbox)
Reference: Exchange 2010: Can You Have Too Much RAM? and Understanding Memory Configurations and Exchange Performance