Shared mailboxwes are not visible by default in Outlook on macOS. You need to add the shared mailbox manually in order to see it.
1. Go to "Tools" and "Accounts..."
2. Click on "Delegation and Sharing."
3. Go to "Shared With Me" and click the plus button in the bottom left.
4. Type in the name or email of the shared mailbox, select it and click "Add."
5. Click on Done.
6. The shared mailbox should now be visible in the left sidebar in Outlook.
