Arbitration mailbox requirement on hybrid exchange servers

Question

We have an exchange hybrid environment where all our mailboxes are in 365. We're decommissioning our existing on-prem Exchange servers are replacing them we 2 new Exchange SE servers. These servers will only be used for recipient management, smtp relay and resolving emails to some dynamic distribution groups that are remaining on-premises as the can't be migrated.

All our email goes to 365 before going on-prem if it can't be delivered to a 365 recipient (for example the on-perm DDGs)

I'm trying to understand if we need a DAG on the new SE servers to host the arbitration mailboxes. Our existing servers have a DAG and the arbitration mailboxes, this is a legacy setup from when we had on-prem mailboxes. This Q&A asks a similar questions, however the answer is vague as to whether the the arbitration mailboxes should remain on a server or whether a new DAG is needed: https://learn.microsoft.com/en-us/answers/questions/389415/arbitration-mailbox-requirement-on-a-hybrid-server

I've also read articles that say we can't remove the old DAG until we've migrated the arbitration mailboxes. The question is, where do these get migrated to? A new DAG on the SE servers or a database on one of the two servers?

Answer 1

Hi Jon Kilner

To directly answer your question: No, a DAG is not required on your new Exchange SE servers to host the arbitration mailboxes.

Technically, in a hybrid environment where all user mailboxes are in Microsoft 365, the DAG you currently have exists solely for user mailbox resilience, which is no longer needed. Arbitration mailboxes must still be retained (their absence causes EAC and Exchange Management Shell failures), but they do not require DAG-level protection. A single-copy mailbox database on one of your two SE servers is entirely sufficient and hosting them there does not conflict with the hybrid license, as its user mailbox restriction does not apply to system mailboxes.

Additionally, the best practice is migrate the arbitration mailboxes to a plain, single-copy database on one of the new SE servers.

You can use New-MoveRequest to move them across (Migration mailbox first), verify all system mailboxes reflect the new database, then dismantle the old DAG and decommission the legacy servers. And please do not decommission the old DAG before the migration is complete.

You can read here for more information:

Re-create missing arbitration mailboxes

https://diecknet.de/en/2023/05/16/exchange-eac-not-working-after-decomissioning-all-databases/

Note: This information is provided as a convenience to you. These sites are not controlled by Microsoft, and Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please ensure that you fully understand the risks before using any suggestions from the above link.

Hope my answer will help you.

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