To successfully complete the Exchange backup directly to a NAS, or to any network path, the users of the group Exchange Trusted Subsystem must have write permissions for the backup folder.
If this is the case, with a NAS joined to a domain you just have to give access of the backup folder to the group.
Othwerwise, you can store the Exchange backup locally and check the field Copia il Copy backup in the common destinations in the backup Wizard. Add the NAS as a common destination and the Uranium Backup service – with its user – will copy the data there.
Feature available in all Uranium Backup commercial editions..