Open a Shared or Delegated Mailbox on Apple Mac

Use this method if you are a delegate for another user’s mailbox or when you have permissions to several folders in a shared mailbox. You can also use this method when you have full access permissions to a mailbox.

To open the shared mailbox:

1. In the Tools menu, choose Accounts, and select the account that has access to the mailbox.
2. Select Delegation and Sharing
3. Choose Shared With Me tab
4. Choose + to add a shared or delegated mailbox

Full article