Exchange Hybrid Duplicate Mailboxes

Some customers have seen where they can end up with a duplicate mailbox within their hybrid environment. This can cause problems when a user, either on premises or in the cloud, has two mailboxes, one on premises and another one in Exchange Online (EXO). The good news is, there is now a work around.

A new parameter that can be called by using the Set-User cmdlet in Exchange PowerShell was introduced recently to help resolve this occasional issue that’s seen when in a hybrid Exchange environment that resulted in duplicate mailboxes.

To set the user Jon, in the Contoso domain, to be a clean object that can be re-licensed without causing the two-mailbox problem, run the command:

Set-User -PermanentlyClearPreviousMailboxInfo

Once you’ve run the command, you can on-board Jon’s on-premises mailbox to the cloud as usual. For additional information you can review this eHelo blog post:

However, you do have to remember, that duplicate mailboxes could be a natural and intentional item. Which is why there is logic within EXO to resolve this conflict during the provisioning process. Additional information posted here that explains this shared mailbox concern:

So there you go, two scenarios that can result in a duplicate mailbox, either by error, or by design, and how to fix each given situation.

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