Gmail confidential mode
Problem
Google's Gmail web application allows users to send emails in "confidential mode". This means that the email expires after a configured time period, and the sender can revoke access to it at any time.
JEMH (or any other non-Gmail system) cannot access the contents of the email when confidential mode is used. Instead, an email with a secure link is provided:
Solution
The email sender will need to avoid using this mode when sending to JEMH in order for the content to be present for processing.
Email server administrators can potentially catch this scenario early by setting a rule for emails containing the X-Gm-Locker header.