There are several issues that you must be aware of before implementing this. You should ensure that your users know of your ability to snoop around thier mailboxes, and that you have read all relvant legal documentation.
I’m assuming that you require access to something similar to the service account in Exchange 5.5.
This is relativley easy to do an there is a MS KB article on how to do this. The Article ID is KB262054 and can be found at: http://support.microsoft.com/kb/q262054/
In a nutshell:
1. Create a group in AD Users & Computers called something like “Read All Users E-Mail”
2. Add the required user accounts to the group, making sure that you only provide those who need to read EVERYONE’s mail
3. Open up Exchange System Manager and grant this group access to the database objects that you want them to view. You need to ensure that you have full access to the mailbox.
you may also need to read http://support.microsoft.com/?id=264733
There is a quicker way of doing this, but it relies on you having deployed the Exchange Active Directory Connector. If find that the above solution works better in my location and my situation, since I can add members of my team and remove them without messing about too much.
I have run this on my exchange server (it’s in a school, so monitoring is important) and it’s worked both on 2000 and after I migrated to 2003.
Also, depending on your situation, you may wish to investigate third party products to monitor e-mail. I’d suggest looking at msexchange.org
Hope this helps.