Hi, Is it possible to view all messages stored in users' mailbox as an administrator without changing their password? Thanks

It is, but you have to override some default deny permissions in exchange using ADSIEDIT. http://support.microsoft.com/default.aspx/kb/292509should get you going...i think this article references 'send as' and 'recieve as' permissions, but you'll need 'full mailbox access' permission as well.