I just included it as a default case. If you want to assume malintent, that's on you. The more interesting cases are the last two because they reveal that messaging cannot be made private because you cannot prevent the counterparty from leaking information.
Could you clarify whom you hope that a message should be able to be read by?
- the intended recipient(s)
- parties involved in the intermediate storage system
- middleboxes / parties in the transmission path
- unintended recipients that the intended recipients forwarded the message to accidentally
- unintended recipients that the intended recipients forwarded the message to maliciously