Dear Peer_py
Thanks for choosing Microsoft Community.
Based on your description, I know the scenario you met. For this, I understand how frustrating it can be to lose access to your shared mailboxes. Here's how you can add them in the new Outlook for Windows:
- Right-click your account name in the Folder pane on the left side.
- Select "Add shared folder or mailbox" from the context menu.
- In the window that appears, type the name or email address of the shared mailbox you want to add.
- Click "Add".
The shared mailbox should now appear under the "Shared with me" folder in the Folder pane. If it doesn't show up immediately, try restarting Outlook.
If you want to add the shared mailbox to your favorites for easier access:
- Locate the shared mailbox in the Folder pane.
- Right-click on the Inbox of the shared mailbox.
- Select "Add to Favorites" from the context menu.
Here is the reference: Open and use a shared mailbox in Outlook - Microsoft Support.
Hope this can help. If there is any update, please feel free to update me. I will continue to help you based on the information you provide. Your understanding and co-operation are highly appreciated.
Thanks for your precious time. Have a nice day.
Sincerely
Katherine