A domain alias is an alternate name for an Internet domain.
The name of a user’s domain appears after the @ symbol in his or her email address. By adding a domain alias, you give every user in your domain a second email address (with the alias after the @). For example, if your domain is mywebsite.com and you add mywebsite.net as a domain alias, every firstname.lastname@example.org will also receive mail addressed to email@example.com.
There are two main reasons you might want users to have multiple email addresses:
•For branding purposes. If your organization uses different names as part of its brand identity, you might want your users to have email addresses reflecting the different names.
•To route email to different email inboxes. A user can have a separate inbox for each email address. This routing option is useful for configuring dual delivery, where a user receives mail both in Gmail and in the legacy email system.