Aside from users having either delegate or ACL access to a mailbox in exchange 2003, are their any other risks associated with exchange mailboxes? Can you provide some examples, and any best practice that should be followed to mitigate these risks?

Everything can be a risk. You need to do your best to address is. For e.g, if you haven't got backups in placem then there's a risk that you'll lose mailbox data. Best practise is to perform backup (Full would be preferred if possible) If you haven't configured exchange properly that can be a risk itself. As a minimum/best practise, use the ExBPA an make sure all issues are corrected as appropiate. Make sure you have configured your limits for each database for deleted items/mailbox rentension and selected the option not to permenanetly delete mailboxes and items until the store has been backed up. Make sure you have reasonable mailbox limits otherwise you will find yourself unable to manage your exch servers and will impact admin such as backups/restores/ disk space issues. Maybe, have a mailbox manager policy to delete items in deleted items folder to manage mailboxes (maybe consider toher folders too if appropiate).Sukh