If you are a delegate for another user’s mailbox, you can get access in Outlook for Mac by following the steps below.
From the top bar> go to Tools menu, choose Accounts, and select the account that has access to the mailbox.
Select Delegation and Sharing.
Go to Shared With Me tab.
Choose + to add a shared or delegated mailbox
If you have permissions to custom folders in the mailbox but not to the top-level mailbox folder, the custom folders will not appear in the folder list.
Only default folders that you have permissions to, such as Inbox, Sent Items, Deleted Items, and Calendar, will appear in the folder list without top-level mailbox folder permissions.