If you need to monitor a second Exchange Server mailbox in addition to your own, how can you find out when that mailbox gets new items? We can think of four different methods:
You cannot create rules in your own mailbox that operate on items coming into another user's Inbox. However, if you have Owner access to the mailbox (or the aid of an administrator with Owner access), you can create a new profile to open the other mailbox directly. While working in that profile, create a rule that forwards items from the other user's Inbox to your own. This rule will run on the server. (If you're not sure what makes a rule server- or client-based, see Server-side and Client-side Rules
You can add the other mailbox's Inbox folder to your navigation pane. The other user or an administrator needs to grant you Reviewer rights on the top level of the mailbox (the one listed as Outlook Today in the Folder List). Then, you need to go to Tools | Accounts, select the Exchange account and click Change, then More Settings. On the Advanced page, add the other mailbox to the service. When you return to the Folder List, you'll see the other mailbox listed along with yours. You can then drag the Inbox to your navigation pane or Outlook bar. The icon should keep an unread message count in parentheses, just as it does with your own mailbox folders.
Another possibility is an Exchange Server Event Service script that sends out a message when a new item arrives in a folder. For an example of such a script, see Public Folder Update Notification. This sample was designed for public folders, but could be adapted to mailbox folders. If you're new to Exchange folder scripts, see Microsoft Exchange Server Scripting and Routing.
We list several notification tools at Mail Notification Tools and know that at least one, MAPInotify, can modify multiple Exchange mailboxes.