Toggling all headers can help, but then you see way too many things. Maybe add a toggle to change fields where it makes sense to a vertical list instead of horizontal?
It came up again on IRC, copying some thoughts from there for the record:
rockorager:
Here's a thought...a new command :envelope which pops up a dialog that has it all laid out a little nicer Could have every recipient on it's own line then and have it vertically scroll if needed
ferdinandyb:
another thought: what gmail does for example is to only display first names by default I could imagine a default format specifier for TO,FROM,CC,BCC fields + a popup
Koni Marti referenced this ticket in commit 19e2750.