Symptoms
When you run the Disable-Mailbox cmdlet in Microsoft Exchange Server 2016, you might find that the LegacyExchangeDN attribute can't be removed from the user's properties in Active Directory.
This issue occurs after you install Cumulative Update 4 for Exchange Server 2016.
Resolution
To fix this issue, install Cumulative Update 9 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
References
Learn about the terminology that Microsoft uses to describe software updates.
