We have one Exchange 2007 on Windows Server 2008 with 32GB memory. There are only 10 user mailboxes and the server report 30GB of memory used, with store.exe using 25GB.
Each user mailbox is about 10GB. Does this point to some problem? is store.exe using 70% of the memory because users have all the emails in the Inbox instead of subfolders? One user had 7,000 emails in Inbox, the rest I suspect behave the same way.