grutch's response hit the proverbial 'nail on the head'. Lotus Notes "NSF" files are proprietary Lotus Notes database files and Outlook is simply an email client, hence the problem. If you (for example) received a lotus notes "nsf" file as an attachment to an email message, you would still need a Lotus Notes client (and a lotus notes ID) installed on the local machine which Windows would then 'associate' the attachment with "notes", open notes, and then open the database (presuming you had access to the file based upon the access control list (ACL).
I've never tried using the too grutch suggested, by I'm interested in finding out how and IF it can work, pending encryption and ACL issues???