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I'd suggest a bit of clarification here.
Exchange operators most likely do backup the server but they are unlikely to backup the personal folders on the users' computers. We shouldn't confuse the two types of backup.
If users moved messages from the OST (which is in synch with the Mailbox on the server) into PST files (which or stored on a local computer), they have to take care of their PST files.
Some organisations create various other types of backup (and archives) on the servers, and these can be sources of recovery if PST files are lost. Don't confuse these additional techniques with the backup of the Exchange servers.