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:

For Exchange Server 2019, install the Cumulative Update 2 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.

For Exchange Server 2016, install the Cumulative Update 13 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.


Learn about the  terminology that Microsoft uses to describe software updates.

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