Change the primary domain
If you connect multiple domains to your organization, one of them will become the primary domain, and the others will be relegated to alias domains.
You can only create employee mailboxes using the primary domain. An alias domain is simply an alternative name for a primary domain. Additional addresses hosted on the alias domain are linked to the mailbox hosted on the primary domain.
If you don't have access to your company's primary domain or don't want to use it, you can transfer the primary status to another domain instead. When you change your primary domain, your organization's mailboxes will be moved to the new primary domain. All data about your mailboxes will be kept intact.
To promote a domain to the primary domain, do the following: