DNS domains

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 in your company is also working within Office 365 and added the domain to their tenant, it will not be possible for you to add the domain until that conflict is resolved. We have witnessed this when someone who tried Office 365 before they left the company and their information was lost; it also happens when customers move from big Office 365 aggregators like GoDaddy or Comcast to a partner who will give them more control over their tenant. In cases like these, you will need help from Microsoft, a partner, and/or the other vendor to restore enough access to the old tenant to remove the domain.