How to define what e-mail for the user?

After addition new account to define e-mail the address it is possible so:
User Name@ Computer Name: Computer Name - is a computer name [domain or full computer name] on which it is installed Gattaca Server, and the given user is registered.
User Name@ Computer Address: Address of the Computer - is IP the address of a computer on which it is placed (installed) Gattaca Server, and the given user is registered.
User Name@localhost: The Given variant approaches, if not usage more than 1-st server in the network is planned.

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