[leephillips]

format=flowed does not solve the problem, but you don’t have to resort to HTML. This guy¹ has the right idea: just make each paragraph in your email a single long line (but no more than 998 characters). The idea that we need to break lines at 78 characters, or whatever, is false. Long lines will be wrapped responsively in any sane client, so the reader will have a good experience.

[1]https://www.arp242.net/email-wrapping.html

[jlokier]

One of the complaints I've had is that my mail had to be horizontally scrolled to be read.

One long line sounds likely to make the problem for that person worse not better!

That was a while ago though. Clients may have changed.

I'd love to know if someone has put the time into cross-platform tests for this sort of thing.

The only thing I'm completely confident of at the moment is that multipart/mixed text+HTML works for every client.