NST files are similar to Outlook Personal storage files. They store data in a similar manner but in .nst format. The NST file data allow a user to access their lotus notes account through outlook. The NST file is access offline using IBM Notes account in Outlook. These files are automatically generated in Outlook for Lotus Notes account.
Since this file is automatically generated by outlook hence there is no direct method to open this file. These files are similar to personal storage files hence may be opened by renaming in that format but make sure you have a backup of these files before renaming it.