There are two ways for one organization to use multiple domain names: with multiple domains or with multiple domain aliases. Which one is appropriate for your organization?
Fundamentally, a domain is an easy-to-remember name for a range of addresses (for computers and users). When you want two names that refer to two distinct ranges of computers and users, you want two domains. When you want two names that refer to the same range of computers and users, you want two aliases for the same domain.
In the graphic below, Parent Company has three domains: parent.com, subsidiaryA.com, and subsidiaryB.com. People in the corporate office have accounts on the parent.com domain while people in the subsidiary business units have accounts on subsidiaryA.com or subsidiaryB.com.
Suppose that Parent Company continues to market some products under a former company name, BD Products. As a result, everyone with an email address @parent.com also has an email alias consisting of the same user name @bdp.com. Because this scenario calls for the same users to have two addresses, the solution is to create a domain alias for parent.com. The graphic below shows the domain alias BDP.com serving as an alternative name for parent.com.
When you associate multiple separate domains with your Google Apps account, you have more flexibility in how you assign usernames. The same username can refer to different users in different domains; for example, [email protected] and [email protected] can be different users. With a domain alias, the same username in both domains must refer to the same user; for example, [email protected] is and must be an email alias for [email protected].
A user’s primary email address is the email address assigned when the administrator creates that user’s account. A user may have additional email addresses as email aliases, but his or her primary email address has a special status: it is the address the user enters to log into Google Apps, to sync with his or her mobile device, and to share Google Drive files and Sites. With multiple separate domains, a user’s primary email address can be in any domain associated with the Google Apps account; with a domain alias, the address in the alias domain is an email alias and never the primary email address.
