Microsoft 365 shared mailbox not showing up in Outlook on some clients

Question

We've just started migrating to M365. I have my hybrid connector established and we've migrated myself and the other IT staff and things appear to be working normally as we prepare to migrate more users.

I created a shared mailbox in Exchange Online (where my mailbox is) and gave myself full access to it. The problem I'm experiencing is I can see this mailbox in my Outlook on one machine (which is at work), but not my home machine.

My machine at work is a domain-joined PC running Windows 10 and Outlook 2019. My machine at home is my personal PC running Windows 10 and Outlook 365.

From my machine at work, clicking on the Mail tab in Outlook shows me my mailbox, the shared mailbox hosted in 365, another shared mailbox (that was migrated from on-premises), and shared on-premises mailbox that has not been migrated. I also see Public Folders (which have not yet been migrated, so I'm seeing the "empty" ones that are in 365--this is because I needed to create a public folder mailbox in order to install an organization forms library for our legacy archive solution that will be deprecated post-migration).

From my machine at home I see only my mailbox, the shared mailbox that was migrated, and the shared mailbox that is still on-prem. I don't see the shared mailbox that I'd created in 365, nor do I see any Public Folders.

On my home machine I've tried going into the account settings and telling it to also open the 365 shared mailbox. This causes it to show up in my list, but if I try to expand the mailbox to access its Inbox or other folders, Outlook throws out the error that the set of folders cannot be opened.

As another test I logged into our Citrix environment (another legacy system that we'll be phasing out). Again, I'm logging into the domain, albeit in a Windows 2012 R2 Citrix environment and running Outlook 2016. On that machine I see the migrated shared mailbox, the non-migrated shared mailbox and public folders--still not the shared mailbox that I created in 365.

Is the issue that I'm not logged into my domain? Is that required to access this mailbox and public folders?

I also just realized it's fair (and possibly pertinent) to point out that I was using "shared mailbox" in multiple ways above since "shared mailbox" has a specific meaning in later versions of Exchange and EOL. The 365 shared mailbox was actually created as a "shared mailbox" in 365. The other mailboxes (the ones I can see in all clients) are typical user mailboxes that I've been added to with Full Control and Send As permissions (they're mailboxes that were migrated from our Exchange 2010 environment).

So in summary:

• My mailbox has been migrated from on-prem to 365.
• All my clients can see the standard user mailboxes--both migrated and still on-prem--that I've been granted Full Access to.
• Only my domain-joined PC at work can also see the 365 shared mailbox I created online.
• Both my domain-joined PC at work and my domain-joined Citrix session can see Public Folders, but my home PC cannot.
• I can log into outlook.com and go to "Open another mailbox" and choose the 365 shared mailbox and it opens for me.

Answer 1

Hi @Joe Grover ,
Could you try to assign the “Full Access” permission of shared mailbox to another user and try to open it in Outlook client?
Have you tried adding a Public folder in OWA and opening it?

According to my test, I create a shared mailbox on Exchange online in my Hybrid lab environment and assign the “Full Access” permission to a user. After that, I log in to the user’s mailbox on the Outlook client on the domain-joined and no domain-joined machines. I could see the shared mailbox on both machines. So I think the issue has nothing to do with whether your machine has been joined domain.

According to the shared mailbox, it can be opened on OWA, but not on the Outlook client. Please try to following the steps:
1.Please check your DNS records and make sure that correct Autodiscover record is configured.
For more information: Microsoft Remote Connectivity Analyzer
2.Please check whether the shared mailbox was assigned with the Exchange online license. And run the following command to check whether enable the MAPI for the shared mailbox.

Get-CASmailbox –Identity <>  

3.Please try to create a new outlook profile on the machine at home. Then please try to type the “outlook.exe /safe” in the Run to start outlook as safe mode and see if the issue has been resolved.
4.Please run the following commands to remove the “Full Access” permission and add it again, but please note that set the “AutoMapping” to False, then manually add the shared mailbox in Outlook client.

Remove-MailboxPermission -Identity <Shared mailbox> -User <User Mailbox> -AccessRights FullAccess  
Add-MailboxPermission -Identity <Shared mailbox> -User <User Mailbox>  -AccessRights FullAccess -AutoMapping $false  

---

If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.