@Andy David - MVP
Yes i went through the article prior to reach to community. Microsoft doesn't articulate the downstream impact if UPN doesn't change to SMTP. For an organization to put an effort and spend money on to change UPN need valid justification which is missing from Microsoft articles.
"Reason? This way they know their logon name since it matches the primary.- This is mainly for user inconvenience" this doesn't help for an organization to change all LOB applications authentication mechanism.
"Also, this ensures no duplicate conflicts down the road with other accounts.- We have Empid as the UPN prefix and the public routable domain name suffix. Hence Empid won't be duplicated at any cost." How i will justify with these assumptions for an organization to spend money to change the authentication mechanism. "
In addition to that, UPN suffix on On-prem AD is publicly routable DNS name which has been verified on M35. Both SMTP and UPN suffix domain names are same. We can't consider email as an alternative login as PC are moving towards to Hybrid domain joined.
Do you have MS or any other articles which clearly articulate UPN mismatch to SMTP downstream impact to M365 services such as EXO, SPO, Teams and Etc?