In an environment with multiple mail servers, the Class of Service is 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. you can limit the servers available to the pool for the COS. When a COS is assigned to a new account, a mail server is randomly chosen for the new account from the server pool list associated with that COS.
Go to Configure>Class of Service.
Double-click on the COS to edit and select edit the Server Pool page.
On the Server Pool page, add a check mark to the servers that should be available for creating new users.
Click Save.
Note: If you select Do not limit server pool available for creating new users in this COS, all servers are in the server pool.
Go to Manage>Accounts.
In the gear icon, select New.
On the General Information dialog box, in the 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.
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