Configure Active Directory to Support Additional Domain Names
Add an alternative UPN suffix to your AD forest so users can sign in as user@infotechninja.com without renaming the domain. Walks the optional internal DNS zone, the AD Domains and Trusts registration, the per-user assignment, and the Microsoft 365 hybrid pre-flight (domain verification, UPN audit, sign-in mode). Includes the Set-ADForest / Set-ADUser PowerShell equivalents and the common mistakes (wrong right-click, suffix vs email, forgetting public DNS validation).