To check if we should use the plaintext body modification API in Outlook or not we use:
This is incomplete and might cause users not to be able to view the mail content as our modifications to the HTML Body are rejected even though we should handle this and fall back to plaintext in case of error. This part needs to be checked again if the error handling somehow regressed in recent versions.
Additionally we need to at least check:
I am also pretty sure that we should additionally check HKLM in case the HKCU value is not set.