How can I grant a user access to someone else's mailbox in Microsoft Outlook?

Log on to Outlook as the user who owns the mailbox and select Options from the Tools menu.

Select the Delegates tab.

Click Add.

You'll see a list of users who can be given access to the mailbox. Select the user or users to whom you want to delegate access and click Add, as the figure shows. Click OK.

Now select the rights that this delegated user has. By default, users are given only Calendar and Tasks access, but you can give access to other areas (e.g, Inbox, Contacts) at different levels (e.g., Reviewer, Author, Editor) as the figure shows. Notice you can also select the option to send the delegate an email message confirming the access level and the option that sets whether the delegate can see items marked private. Click OK.

Click OK to the main Options dialog box.