Add a toggle to show full headers in message view
Add a toggle or clickable place (like the "see more" in apple Mail) that allows you to reveal the full list of addressees when viewing a message, rather than the popup box that shows all the headers in some strange non-intuitive fashion.
This shouldn't even be a question. Why is this not in-product?
I agree. In the age of spam and fraud, full headers are essential. Toggle would add the convenience of quickly showing the former when necessary.