I was given the Exchange Admin position and asked to cleanup the permissions on the public folder. Many users have left the company and this is one task that has been neglected. While trying to clean it up, I found several users that show up as NT User:S-1-5-21-xxxxxxxxx where x could be anywhere from 30 to 35 numbers. Is this some kind of orphaned SID of a user that has since been deleted from Active Directory? Would it cause a problem to delete this user from the folder? Or could this be a username for some application that needs access to the public folder?