You need to check if an Office 365 tenant domain name is available or already registered by someone else.
- Make use of the following service: https://o365.rocks/. There you will be able to do a quick check for the availability of a tenant name in Office 365.
- Make use of the PowerShell script available at: https://www.linkedin.com/pulse/how-check-office-365-tenant-name-availability-aaron-dinnage
- Make use of the Powershell script available at Microsoft Technet Gallery: https://gallery.technet.microsoft.com/Check-domain-registration-2fb6d097. This script can be used to determine, if a domain is already registered or verified in an Office 365 or Azure Active Directory tenant or if it is not used in any tenant.The purpose is to make troubleshooting for user login errors in Office 365.