Forum Discussion
Official recommendation to UPN equal to SMTP/email address
I don't think there's anything "official" official. It's mentioned as "best practice" in multiple articles, for example here: https://docs.microsoft.com/en-us/office365/admin/add-users/change-a-user-name-and-email-address?view=o365-worldwide#tip-keep-the-persons-old-email-address
In reality, it depends on the workload and the client app. Some of them have a proper understanding of the difference between UPN and SMTP address, others "assume". Microsoft does enforce it for some endpoints though, for example when making changes via the O365 Admin Center. They also have a requirement that at least one of the smtp addresses should match the UPN in O365 (not necessarily the primary one though).
Dean_GrossI think this may be the reference you are looking for:
A UPN is an Internet-style login name for a user based on the Internet standard RFC 822. The UPN is shorter than a distinguished name and easier to remember. By convention, this should map to the user's email name. The point of the UPN is to consolidate the email and logon namespaces so that the user only needs to remember a single name.
https://docs.microsoft.com/en-us/windows/win32/ad/naming-properties#userprincipalname