I had complains about "too long lines" in my e-mails many times and now try to use EOLs, unfortunately.
It is exactly what I said: all modern, GUI e-mail clients suck at basic functions, like displaying simple message with several layers of quotations and long lines, they cannot wrap lines with visually duplicating quotations properly, often they could not wrap even "new" (unquoted) lines at all and show horizontal scrollers. JMAP is no help here.
It was solved problem in age of terminal clients, heck, GoldEd for FIDOnet was able to do this in 1990s.
Instead of new protocols it was better to standardize MarkDown (+extension for nested quoting) in e-mails, but this train is long gone, I'm afraid.
