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Hi @Niclas Chmill
Thank you for reaching out with your question about managing Exchange Online recipients after decommissioning your on-prem Exchange 2016 server. I really appreciate the time you took to research this topic carefully, it’s an important consideration in a hybrid setup, and I can understand wanting a clear and supported path forward.
As a forum moderator, I'm here to help clarify things and point you toward reliable resources from Microsoft.
From what I've found, once all mailboxes are migrated to Exchange Online, the Exchange 2016 Management Tools are not supported for standalone use after the last on-prem Exchange server is removed. They require an active Exchange environment to function properly.
Microsoft does offer a supported solution starting with Exchange Server 2019 CU12. You can install the Exchange 2019 Management Tools in "Recipient Management" mode on a domain-joined workstation or server. This allows you to manage mail-enabled users, groups, and contacts in Active Directory, even after decommissioning your on-prem Exchange server, as long as directory synchronization is in place.
Here are a few key points you might want to review:
- Your environment needs to be using Microsoft Entra Connect (formerly Azure AD Connect) to sync on-prem AD to Microsoft 365.
- Recipient updates should continue to be made using Exchange cmdlets, not directly via AD tools.
- Managing synced (hybrid) recipients purely from Exchange Online PowerShell isn’t supported, as some attributes still need to be set in AD.
For more detailed guidance, here are some links to the official Microsoft documentation that can help with your planning:
- Decommission on-premises Exchange servers
- Hybrid deployment prerequisites
- Exchange Server hybrid deployments
- Manage recipients in Exchange Hybrid environments using Management tools
- Directory sync with Microsoft Entra Connect
- About the Exchange Online PowerShell module
Based on this documentation, the typical next steps after your mailbox migration would be to install the Exchange 2019 Management Tools, then safely decommission your on-prem Exchange 2016 server. This provides a clean, supported, and streamlined management solution.
I hope this helps clarify the direction a bit. Thanks again for bringing this topic up. I hope the resources above are helpful as you move forward!
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