"Outlook Web Access does not allow you to view the message headers for an email message, however, thanks to some crafty programming by Sig Weber you can add this functionality easily.
To enable this feature, you need to download Sid's modified util_View.js file and replace the original located on the Exchange server under \Program Files\Exchsrvr\exchweb\6.5.6944.0\controls. Remember to back up the original first!!!
Once you have copied the new util_View.js file, you will need to restart IIS. Once that is complete a View Source option should appear in the right-click menu for the message."