In an environment with multiple mail servers, the Class of Service can be used to assign the next account to a mailbox server or you can specify a specific server when you create the account.
In the COS, the Server Pool lists the available mailbox servers, and you select which servers should be available or the pool. When a COS is assigned to a new account, a mail server is randomly chosen for the new account from the server pool associated with that COS.
To add a server to the server pool
Open a COS or create a new COS, and select the Server Pool tab.
Click the server name from the left column.
Click Add, to move the name to the right.
Click Save, to commit your changes.
To add an account to a specific mail server
In Accounts, select New Account.
On the General Information dialog, in the Mail Server field uncheck auto.
From the drop-down list, select the mail server for the new account.
Continue to create the account and click Finish.