In Microsoft Exchange Server 2019 or Exchange Server 2016, consider the following scenario:
User 1 is an Active Directory (AD) user whose UPN is as same as User 2's SMTP address.
User 2 is a mailbox user who has different UPN and SMTP addresses.
You try to impersonate User 2 by using his primary SMTP address.
In this situation, the impersonation fails, and you may receive the following error message:
The primary SMTP address must be specified when referencing a mailbox.
To fix this issue, install one of the following updates:
Learn about the terminology that Microsoft uses to describe software updates.