Mail service acts very quickly to transfer information when users read or send messages, putting intense but brief demands on a server. As long as other services don’t place heavy, continuous demands on a server, the Mail server can typically handle several hundred connected users. If you need to improve Mail service performance to meet increased demand, try the following:
Move the Server app’s data store location to a dedicated hard drive.
Use a different server for other services, especially services that place frequent heavy demands on the server.