Just as you might have an assistant who helps you manage your incoming paper
mail, Microsoft Outlook provides similar functionality by making it possible
for you to give another person access to your Inbox and any other Outlook
folder you want.
By default, if you grant someone access to your folders, that delegate has
access to the items in the folders, except items marked private. You must
grant additional permissions to allow access to private items.

Note  If you want to use the Delegate Access feature, your mail must be
delivered to your mailbox (mailbox: Location on a Microsoft Exchange server
where your e-mail is delivered. Your administrator sets up a mailbox for each
user. If you designate a personal folder file as your e-mail delivery
location, messages are routed to it from your mailbox.) on the server, not to
a personal folders file on your hard disk.