RPNSG files are encrypted messages for Outlook that can only be viewed by the intended recipient, which means that it is not something you can just convert to other format. If you are the intended recipient, you can open the file using Outlook and export it (for example the contacts) or forward it to another e-mail address.
These files are typically found as e-mail attachments, so if they are in your own e-mail, it is likely you are the intended recipient.
Updated: September 13, 2023