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

Original KB number:   2710029


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 Microsoft 365 Hybrid Configuration wizard that's available at For more information, see Microsoft 365 Hybrid Configuration wizard for Exchange 2010.


Assume that you have a hybrid deployment of Microsoft Exchange Online in Microsoft 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 Microsoft 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'


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, Microsoft 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.


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


    The TargetAddressDomain value represents your coexistence domain (for example,

  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.


In the following versions of Exchange Server, you can use the New-RemoteMailbox or Set-RemoteMailbox cmdlet to set the Type parameter of the mailbox to Shared:

  • Exchange Server 2013 CU21 or later
  • Exchange Server 2016 CU10 or later
  • Exchange Server 2019

For more information, see Cmdlets to create or modify a remote shared mailbox in an on-premises Exchange environment.

If you aren't running these versions of Exchange Server, or you continue to receive the remoteMailbox.RemoteRecipientType must include ProvisionMailbox error when you use the value Shared, use the Set-ADUser cmdlet.


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.

Still need help? Go to Microsoft Community or the Exchange TechNet Forums.