DNS domains, managing external guest users, and integrating with on-premises user directories are topics that are somewhat more esoteric than the ones we've already covered, and not every organization will have a need for them but for organizations that do need them, they can be very important. We'll cover these subjects in this section.
While DNS domains aren't exactly like users and groups, they haven't been covered elsewhere. Without a custom DNS domain, all your users will end in @tenantname.onmicrosoft.com. Since domains have a direct impact on the user names you can provision, we'll review here what it takes to add a domain to Office 365 and configure it so that you can use it as the suffix for login names and email addresses.
An important caveat: domain names in Office 365 are first come, first served and must be unique across the entirety of all Office 365 tenants worldwide. Therefore, if you added a domain to an old trial account or some other department...