I know that the UPN should be set to the same value as the email address for many reasons, but I can't find the official documentation from Microsoft where they recommend this. Can someone please point me to it?
@Dean Gross Haven´t found any more in-depth statement. But in the article about Alternate ID there is a note stating: "Microsoft’s recommended best practices are to match UPN to primary SMTP address. This article addresses the small percentage of customers that cannot remediate UPN’s to match."
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).
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Dean Gross (Respected Contributor)