Shared mailboxes are unexpectedly converted to user mailboxes after directory synchronization runs in an Exchange hybrid deployment

Note The Hybrid Configuration wizard that's included in the Exchange Management Console in Microsoft Exchange Server 2010 is no longer supported. Therefore, you should no longer use the old Hybrid Configuration wizard. Instead, use the Office 365 Hybrid Configuration wizard that's available at http://aka.ms/HybridWizard. For more information, see Office 365 Hybrid Configuration wizard for Exchange 2010.
PROBLEM
Assume that you have a hybrid deployment of Microsoft Exchange Online in Microsoft Office 365 and on-premises Microsoft Exchange Server. After directory synchronization runs, mailboxes that were moved from the on-premises environment to Exchange Online and converted to shared mailboxes are unexpectedly converted to regular mailboxes. When this occurs, you may be unable to convert the mailboxes back to shared mailboxes. If you can make this conversion, the mailboxes may revert to user mailboxes the next time that directory synchronization runs.

If a license is not assigned to the user, the mailbox may be disconnected. This can cause potentially irrecoverable data loss.

Additionally, when you view the properties of the shared mailboxes in the Office 365 portal, you may receive the following error message:

Exchange: Couldn't convert the mailbox because the mailbox <guid> is already of the type 'Regular'
CAUSE
This issue occurs if the mailbox was migrated to Exchange Online as a regular user mailbox and later converted to a shared mailbox through the Exchange admin center in Exchange Online. In this situation, directory synchronization replicates the attributes to the cloud. Then, Office 365 converts the shared mailbox to a regular UserMailbox object. If the user isn't licensed, and if the 30-day grace period has ended, the mailbox is disconnected and converted to a MailUser object.
RESOLUTION
To resolve this problem, move the mailbox back to the on-premises Exchange environment, and then convert it to a shared mailbox. To do this, follow these steps:
  1. If the mailbox in Exchange Online has been disconnected, temporarily assign a license to the user. This automatically reconnects the mailbox.
  2. Move the mailbox back to the on-premises Exchange environment. To do this, create a new migration batch and select Move from Exchange Online as the batch type.
  3. Convert the mailbox to a shared mailbox. To do this, open the Exchange Management Shell, and then run the following command:
    Set-Mailbox –identity <mailbox identity> -Type Shared
    Note When the mailbox is converted to a shared mailbox, the user account that's associated with the mailbox is disabled. Directory synchronization is set up by default to include disabled accounts. However, if this configuration is changed, it can cause the user account to be deleted from Exchange Online.
  4. Move the shared mailbox to Exchange Online, and then remove the license.
MORE INFORMATION
This behavior is by design when directory synchronization is configured. To correctly create a shared mailbox in this scenario, create the shared mailbox in the on-premises environment first, and then migrate it to Exchange Online as a shared mailbox.

You can't use the Set-RemoteMailbox cmdlet to set the Type parameter of the mailbox to Shared. Therefore, you must move the mailbox back to the on-premises environment in order to convert the mailbox.

Still need help? Go to the Office 365 Community website or the Exchange TechNet Forums.
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Article ID: 2710029 - Last Review: 04/25/2016 11:04:00 - Revision: 18.0

Microsoft Exchange Online, Exchange Server 2016 Enterprise Edition, Exchange Server 2016 Standard Edition, Microsoft Exchange Server 2013 Enterprise, Microsoft Exchange Server 2013 Standard, Microsoft Exchange Server 2010 Enterprise, Microsoft Exchange Server 2010 Standard

  • o365 o365022013 o365e o365m o365a hybrid KB2710029
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