A domain may have one or more Aliases. A Domain Alias inherits all the settings and mailboxes of its parent domain.
Criteria: The domains have the same, exact list of addresses and the same delivery server. If both of these are not true, you cannot alias one domain to the other.
This is very useful when you have several variations of a domain. For example, perhaps "mailroutedemo.com" is your main domain, but you've also registered "mailroutedemo.net", "mailroutedemo.org", and you want them to be configured exactly the same as "mailroutedemo.com" - just make them aliases! Then one user can have email@example.com, firstname.lastname@example.org, etc.
Edit a Domain Alias by clicking on its name. Delete it with the red X. (Important: this will delete all associated settings and email accounts.)
Click the Add Domain Alias button to add a new Domain Alias.