Hybrid Server Removal post CU12

ADPrep Requirement during Setup of Exchange 2019 Management Tools

If you're being prompted to run ADPrep when trying to install Exchange Management Tools, this indicates that your Active Directory schema may need to be updated or that Exchange needs to ensure that your AD schema is ready for the Exchange version you're installing (CU12). ADPrep is typically required when you're preparing for a new Exchange server version or updating an existing one to ensure that the AD schema is compatible.

Solution:

• Running ADPrep after the cleanup script can be a bit tricky, as it sounds like you want to avoid reversing the cleanup steps. If your Exchange 2019 CU12 setup is prompting for ADPrep, it suggests that something in the schema or AD configuration is still pointing to the need for an Exchange server presence.
• You can try to run ADPrep in a controlled way to ensure the schema is compatible with CU12, but you should first carefully assess whether this step is absolutely necessary. It's possible that your schema update was already done during the previous steps, and this might be an artifact of some lingering configuration.

If the ADPrep step is necessary, you might be looking at:

• Running the ADPrep schema update using the adprep /forestprep and adprep /domainprep commands from your Exchange installation media, but be sure to consult your current environment's version and ensure that it's compatible with the cleanup process.
• If you proceed with ADPrep, it might be helpful to run the Hybrid Configuration Wizard (HCW) again to ensure the environment is correctly aligned with the cloud setup and that all objects are cleanly synchronized.

3. Missing Exchange Server for Hybrid Management

Since your goal is to remove the last hybrid server but still manage remote mailboxes and distribution lists (DLs), you are effectively transitioning to a cloud-only management model, but you still need to manage some Exchange-related attributes for on-premises AD objects.

Solution:

• You can still manage hybrid configurations or remote recipients without having an on-prem Exchange server by using Exchange Online PowerShell.
  • Install the Exchange Online PowerShell module (EXO V2 module) and use it for managing remote mailboxes and recipients.
  • Use Azure AD Connect for syncing your on-premises AD objects with Azure AD and Exchange Online.
  • For direct access to Exchange attributes like mailboxes or DLs, you can use Azure AD PowerShell commands.