Not sure if this is the solution.
Most times they forget this if I write after a couple of days or weeks in a existing group.
They get a Mail and simply forget it’s a group.
Probably your right.
If the Mail Client hide the header, of course my friend will not realize the hint.
But if not, hopefully he/she will remember it’s a group.
As I said before: There are much more important things to do, to make DC and regular mail more smooth side by side.
E.G. Reply to Message - #10 by webratte