I would suggest you to go Symantec Backup Exec with the Exchange Agent. You will be able to backup the entire Exchange Server along with the Mailbox stores and the Public folder stores.
You will be able to restore an individual e-mail from the backup without dismounting the Stores.
I use Backup Exec System Recovery and have bought Backup Exec Exchange Recovery which does not
work with Exchange 2007 yet although I have been promised that there will be a release supporting 2007 by
the end of March. At present I cannot restore individual emails and mailboxes easily.
I have also used Devs suggestion and have had very succesful DR test results with both products.
Try using Exmerge: it does brick level backup. It saves individual mailboxes as .PST files.
Once you download the Exmerge for 2003, you need to run it(unzip).
You’ll find an howto in the folder you choose, preferably Exchsvr/bin.
Copy exmerge.dll inside the folder.
Then grant your account or the Administrator account the neccesary rights.
(http://support.microsoft.com/kb/292509/) HOWTO: How to configure an account to use ExMerge on
That works fine and comes at no cost.