I have been asked to look into finding a way to prohibit Outlook from allowing a file attachment file from a certain shared directory. I have 10 users who have read access to the shared directory. They look at files for confidential information. My CEO wants me to find a way to prohibit any of the 10 users from emailing one of these documents through our Office 365 system. They can look at them but the CEO wants to prohibit anyone to send an attachment with any of the documents in that directory. I am so open for ideas how to make this work. USB drives have already been disabled through Group Policy.