My customer has requested us to restore some Exchange 2007 mailboxes but the Exchange 2007 server had been shutdwon and removed long time ago. Basically my understanding is we need at least an Exchange 2007 installed with netbackup, because basically we need to create a Resource Storage group where we can restore the backup db of the Exchange..without a legacy Exchange 2007, there backup is useless and not restorable. Is my understanding correct.
You are correct, the simplest way to be able to perform the restore will be to stand up an exchange 2007 system. This is really a Microsoft limitation as the datbase schema in Exchange changes between versions.
If the backups were taken with the granular recovery option set, you may be able to restore individudal items (but not sure if you still need the Exchange 2007 still). If so the backup has to reside on a supported disk storage device.
I have heard of other solutions, but mostly involve more effort than the supported method above.