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

Applies to: Exchange OnlineExchange Server 2016 Enterprise EditionExchange Server 2016 Standard Edition More
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, regular (user) mailboxes in Exchange Online that were converted to shared mailboxes may unexpectedly be reverted to regular mailboxes. When this occurs, you may be unable to convert the mailboxes back to shared mailboxes. Even if you can make this conversion, the mailboxes may revert to regular mailboxes again 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 RemoteRecipientType attribute is set incorrectly. To be able to convert a User mailbox to a Shared mailbox and not have it be converted back to a User mailbox, the RemoteRecipientType attribute must reflect that the mailbox was migrated or that it is a Shared mailbox. If it does not, 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 when license reconciliation runs.

RESOLUTION

To resolve this issue, make sure that a remote mailbox is provisioned for the account, and then update the RemoteRecipientType attribute. To do this, follow these steps:

  1. If the mailbox in Exchange Online is disconnected, temporarily assign a license to the user. This automatically reconnects the mailbox.
  2. If a remote mailbox doesn't exist for that user, run the following command: 
      
    Enable-RemoteMailbox -Identity PrimarySmtpAddress -RemoteRoutingAddress TargetAddressDomain

    Note The TargetAddressDomain value represents your coexistence domain (for example, alias@contoso.mail.onmicrosoft.com). 
  3. Set the RemoteRecipientType attribute to reflect that the mailbox is a migrated Shared mailbox. To do this, run the following command: 
      
    Set-ADUser -Identity ((Get-Recipient PrimarySmtpAddress).samaccountname) -Replace @{msExchRemoteRecipientType=100;msExchRecipientTypeDetails=34359738368}

     

MORE INFORMATION

This behavior is by design when directory synchronization is configured and the RemoteRecipientType attribute is set incorrectly.
You can't use the New-RemoteMailbox or Set-RemoteMailbox cmdlet to set the Type parameter of the mailbox to Shared. Therefore, in order to set the attribute values correctly, the Set-ADUser cmdlet is required.
Note Under most circumstances, Microsoft does not support the use of non-Exchange tools to manually change Exchange attributes. Because of the limitation in these cmdlets, this behavior is identified as an exception to this support stance.

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