I may comment on this down the trail as I learn enough to be helpful,--
in the meantime I was reading this on HelpNetSecurity:
Clear and present danger: Combating the email threat landscape
I found it interesting to consider the possibilities for Firejail while thinking about this article; as we have all noticed -- just stopping a bad document in the e/mail in-box isn't enough if the document is then copied or forwarded for additional use,--It’s no wonder then, that email represents a major security threat vector. Because, as long as organisations use email to send and receive files, malicious email attachments will continue to plague corporate inboxes. Cyber criminals have consistently proved adept at exploiting the ‘click first, think second’ behaviours of email-users, which have the potential to open the door to malware, or unintentionally expose the business to data loss.