Toggleable format=flowed support

I use Delta Chat to communicate with MUA clients almost exclusively. As far as I can tell, almost all of them do not support format=flowed. It looks like Fastmail’s team also thought through the pros and cons of format=flowed and decided they would not support it.

Given this use-case, I’d like to choose whether to use format=flowed so my mails don’t appear to them to be victims of so-called “embarassing line wrap”.