21st May .
21st May .
"Ability to backup/restore a single mailbox or set of mailboxes such that any mailbox not actively being processed remains online and accessible."
Though you could get creative with mirroring solutions that automatically copy directory contents/or new changes to another server on the OS level, but that just takes care of having backups, not the fact that you have to shutdown the service to restore those backups. (Or run really complicated setups that are way beyond conversation here.)
Don't forget hierarchical storage management also! Auto-move messages to another (usually your slower/older) storage solution after x days/months keeping your nice fast storage space for current messages.
High availability clustering, is also a lifesaver.
Attachment search is also very nice (when I used to send myself PDF's I would also copy/paste the text of that PDF into the email so I could easly search for stuff later; with the network edition you can just send the PDF and the contents will be auto indexed!