Office 365 Hybrid – Shared Mailboxes Created in Exchange Online do not appear to users whose mailboxes exist on local Exchange

I have been working on an Hybrid Exchange Solution using Office 365. The Client has several shared mailboxes and I wanted to see if a Shared Mailbox created in Office 365 would appear for users who have not been migrated.

Long Story short, the answer is no, they cannot. The reason for this is actually pretty simple. Anything created in Office 365 is not assigned a GUID by Active Directory (because we are only doing a unidirectional sync) so Exchange does not have any record of it.

 It turns out however, that if you migrate the mailbox to the On Premise Exchange server a GUID can be assigned.

I found this Article on Microsoft’s support site on how to do it:
https://support.microsoft.com/en-us/help/3129334/users-in-a-hybrid-deployment-can-t-access-a-shared-mailbox-that-was-cr

Here is the process that you would need to follow in order to get it to work

Please note, the website says that you do not need to do this if your are running Exchange 2016 CU 10 or later. The Client was running 13 at the time, however these steps did work.

1. Convert the shared mailbox to a regular mailbox by using the Exchange admin center in Exchange Online. To do this, follow these steps:
   1. Open the Exchange admin center in Exchange Online.
   2. Click recipients, and then click shared.
   3. Select the shared mailbox, and then click Convert.
   4. On the Warning page, select Yes to convert the shared mailbox.
2. Create an on-premises object for the cloud mailbox by using the New-RemoteMailbox cmdlet in the Exchange Management Shell.
   
   Note This object must have the same name, alias, and user principal name (UPN) as the cloud mailbox.
3. Set the ExchangeGuid property on the new on-premises object that you created in step 2 to match the cloud mailbox. To do this, follow these steps:
   1. Connect to Exchange Online by using a remote session of Windows PowerShell.
   2. Use the Get-Mailbox cmdlet to retrieve the value of the ExchangeGuid property of the cloud mailbox. For example, run the following command:

      Get-Mailbox <MailboxName> | FL ExchangeGuid 

   3. Open the Exchange Management Shell on the on-premises Exchange server.
   4. Use the Set-RemoteMailbox cmdlet to set the value of the ExchangeGuid property on the on-premises object to the value that you retrieved in step 3b. For example, run the following command:

      Set-RemoteMailbox <MailboxName> -ExchangeGuid <GUID>

4. Wait for directory synchronization to occur. Or, force directory synchronization.
5. Make sure that the Office 365 user object is displayed as “Synced with Active Directory.”
6. Move the mailbox from Exchange Online to the on-premises environment.
7. Convert the mailbox to a shared mailbox by using the Set-Mailbox cmdlet in the Exchange Management Shell. For example, run the following command:

   Set-Mailbox <MailboxName> -Type Shared

8. Move the mailbox from the on-premises environment to Exchange Online.