Lotus Notes files have an unusual structure, containing both applications and data in a single database-like format called NSF. These files can grow quite big and contain lots of valuable data, so losing them in a data corruption incident may have extremely negative and far-going consequences. When an incident like this renders one or more NSF files inaccessible, specialized tools like NSF Viewer will help view the Lotus Notes content and recover the information from them in the most convenient manner.
For viewing these NSF files, you need a third party NSF viewer tool. Without this, it is difficult to view NSF files.