Exchange Server 2016 allows adding Exchange Server 2019 mailbox server into a same DAG and vice versa

Symptoms

Consider the following scenario:

You separately deploy Microsoft Exchange Server 2019 and Exchange Server 2016 on Windows Server 2019, Windows Server 2016 in the same organization.
You create a database availability group (DAG) and add one of the mailbox servers like Exchange 2016 mailbox server to the DAG first.

Unexpectedly, the Exchange 2019 mailbox server can also be added into the same DAG and vice versa. Two rules are violated here:

After you apply this update, the operation will fail and you will receive the following error message:

WARNING: The operation wasn't successful because an error was encountered.

This issue occurs because Exchange Server doesn't check to disallow Exchange Server 2019 and Exchange Server 2016 to exist in the same DAG.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

To fix this issue, install one of the following updates:

For Exchange Server 2016, install the Cumulative Update 12 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.

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

Keywords: exchange2019 and 2016 can exist in the same DAG, kb, kbfix, kbqfe, kbHotfixAuto, CI97245

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