Installing a first Exchange 2019 to a new Forest, at Setup.exe /PrepareAD, getting an error: Active Directory operation failed on dc01.company1.com. The object 'CN=Microsoft Exchange System Objects,DC=company1,DC=com' already exists.

Question

At Setup.exe /PrepareAD /OrganizationName:"company1" /IAcceptExchangeServerLicenseTerms_DiagnosticDataOn

" was run:
Microsoft.Exchange.Data.Directory.ADObjectAlreadyExistsException: Active Directory operation failed on
dc01.company1.com. The object 'CN=Microsoft Exchange System Objects,DC=company1,DC=com' already
exists. ---> System.DirectoryServices.Protocols.DirectoryOperationException: The object exists.
at
System.DirectoryServices.Protocols.LdapConnection.ConstructResponse(Int32 messageId, LdapOperation operation, ResultAll
resultType, TimeSpan requestTimeOut, Boolean exceptionOnTimeOut)
at
System.DirectoryServices.Protocols.LdapConnection.SendRequest(DirectoryRequest request, TimeSpan requestTimeout)
at
Microsoft.Exchange.Data.Directory.GuardedDirectoryExecution.ExecuteT
at Microsoft.Exchange.Data.Directory.PooledLdapConnection.GuardedSendRequest(String forestName,
GuardedDirectoryExecution guardedDirectoryExecution, DirectoryRequest request, TimeSpan timeout, Func`3
sendRequestDelegate, Int64& concurrency)

We don't have any Exchange server 2019 in Forest,

Answer 1

Hi @Happee ,

Welcome to our forum!

1. Please use ADSI Edit to check if anything else exists in CN=Microsoft Exchange System Objects. For example, there are SystemMailboxes, etc., please delete them.
2. And check Active Directory Users and Computers for duplicate Microsoft Exchange System Objects, and if so, remove the duplicates. Please refer to: ms-exch-setupreadiness-adiniterrorrule

Besides, you could refer to the following similar issue. Make a backup and remove the 'CN=Microsoft Exchange System Objects, DC=company1, DC=com' via ADSI Edit, run PrepareAD again.
the_problem_of_installing_exchange_server_2016

Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.

If methods above do not solve the issue, please check the Exchange setuplog and post the error information.

---

If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Answer 2

This may be related to a previously unavailable domain controller or tasks being run manually in the incorrect order. Or, possibly, the object was created on the backend without POA interference.

Remove conflicting objects using Exchange Management Shell, e.g. for address list: PS> remove-addresslist "S001059833 AL"
That can also be a Global Address List. In that case: PS> remove-globaladdresslist "S001059833 GAL"

Restart failed task. Exchange hosting will be recreated from scratch. GAL and AL will be recreated during the task execution.

Also, check these thread for help - https://learn.microsoft.com/en-us/answers/questions/774581/ms-exchange-2019-install-error-active-directory-op.html

Answer 3

My mistake with I've manually created the "Microsoft Exchange System Objects" in the new forest Active Directory Users and Computers, once I deleted the "Microsoft Exchange System Objects" I was able ran Setup.exe /PrepareAD /OrganizationName:"company1" /IAcceptExchangeServerLicenseTerms_DiagnosticDataOn and installed Exchange 2019 successfuly.