Users in a hybrid deployment can’t access a shared mailbox that was created in Exchange Online [Resolved]

• Date: 24/04/2018 10 minutes to read
  

  Applies to: Exchange Online, Exchange Server 2016, Enterprise Edition, Exchange Server 2016, Standard Edition, Exchange Server 2013 Enterprise Exchange
  

CAUSE

These issues can occur when the shared mailbox is created by using the Exchange Online management tools. In this situation, the on-premises Exchange environment has no object to reference for the shared mailbox. Therefore, all queries for that SMTP address fail.

Microsoft SOLUTION

The following is the Microsoft solution

Which is workaround not a solution because the azure sync isn’t replicating both ways it replicates one way from on-prem to online

So the workaround is to create the same ExchangeGuid property on the new on-premises object to match the cloud mailbox

Move it back to online this way you will have the SMTP both ways

On GAL and OAB for both users online and on-Prem for more details go to the following:

https://support.microsoft.com/en-us/help/3129334/users-in-a-hybrid-deployment-can-t-access-a-shared-mailbox-that-was-cr

It is useful sometimes

https://profadmins.com/2018/04/24/users-in-a-hybrid-deployment-cant-access-a-shared-mailbox-that-was-created-in-exchange-online-resolved/#more-2135

Another solution
I think it is a very good one 

Is creating the online shared mailbox it must be @yourdomain.onmicrosoft.com Then add a contact on your prem which will point to the online shared mailbox

This way both users will be able to see the account also send mail to the shared online mailbox, the only cons for this is that you will receive duplicate SMTP address warning in the office 365

GAL AND OAB will be both updated

Note:

GAL Is syncing from the Global catalog if you aren’t using cache mode OAB is used by online and cached users

First, try to create it using the admin center portal

https://outlook.office365.com/ecp

Create the shared mailbox must be

@yourdomain.onmicrosoft.com as primary

Then go to you

Exchange admin center on-prem

Open contacts start creating mail contact with external SMTP

Create new contact choose alias you want and display name this will appear for both on-prem and online

Add the external email address of the shared mailbox which will match the one you have created on the admins center for online shared mailbox

Then go to you Azure ad connect server

Open the azure ad module for windows power shell as admin

Ru the following command

Start-ADSyncSyncCycle -PolicyType Delta

Then open the synchronization service

Go to operations

Make sure the updates are there

You will have the following error in the admin center you can ignore it

Just telling you that you have duplicate SMTP and this is something you know because you are the creator