New Outlook Shared Mailboxes disappearing randomly

Hi @Raddo, 

Thank you for posting your question in the Microsoft Q&A forum.      

Based on the details you shared, I understand that shared mailboxes in the new Outlook can disappear intermittently for users, even though the same behavior was previously seen in classic Outlook and prompted the move to the new experience. I truly understand how disruptive this can be for day to day operations, and I appreciate the time and effort you have already spent validating the issue across users and clients. 

This behavior can occur when the new Outlook refreshes authentication in the background and then reloads mailbox connections from the service, because the shared mailbox list may not consistently repopulate during that cycle. In addition, when access relies on automapping and there is a delay in permission synchronization, the mailbox can temporarily stop displaying even though access remains assigned. As a result, the mailbox may reappear only after the client completes a successful refresh of its account and mailbox configuration. 

To improve stability, please try the following options in order, since each one strengthens the shared mailbox connection in a different way. 

1/ Add the shared mailbox explicitly in the new Outlook 

• Open the new Outlook and select Settings. 
• Go to Accounts, then Email accounts, and select the user’s primary account. 
• Select Manage, then locate Shared mailboxes, and choose Add shared mailbox. 
• Enter the shared mailbox email address, save the change, and restart the new Outlook. 
• Sign out once, fully close the app, then sign back in to confirm the mailbox remains visible after a full refresh. 

2/ Refresh shared mailbox permissions to republish the association 

• In the Exchange admin center, confirm the user has Full Access to the shared mailbox and, if needed, Send As or Send on behalf permissions. 
• Remove Full Access for the user, wait 10 to 15 minutes, then add Full Access back. 
• Ask the user to sign out of the new Outlook, fully close the application, then sign in again. 
• If access is granted through a group, verify the user’s membership is current and that directory synchronization has completed. 

3/ Repair or reset the new Outlook app state to rebuild mailbox connections 

• Ask the user to sign out of the new Outlook and close it completely. 
• In Windows, open Settings, select Apps, then Installed apps, locate Outlook (new), and open Advanced options. 
• Select Repair, reopen the new Outlook, and check whether the shared mailbox remains present. 
• If the mailbox still disappears, return to Advanced options, select Reset, then sign in again and recheck the shared mailbox list. 

4/ Contact IT administrator:  

Since your account is managed by your organization, please contact your IT administrator to review your permissions and policies. If the issue persists after these checks, ask your IT administrator to submit a support request directly to Microsoft Support team.       

They can raise a support ticket by visiting: Get support - Microsoft 365 admin | Microsoft Learn 

As community moderators, we appreciate your understanding that our access to internal development details is limited. Our primary role is to guide users toward the appropriate resources and support channels. While we may not have visibility into deeper backend analysis, we’ll continue doing our best to support you within the scope of our responsibilities.     

I hope this information is helpful. Please follow these steps and let me know if it works for you. If you have any updates regarding the issue, please feel free to share them with me.       

Thank you for your patience and your understanding. I look forward to continuing the conversation. 

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