It seems that disabling html formatting for outgoing messages, is not that clear. Most probably all you have to do, is select the whole message and then click on the “clear formatting” button [Tx].
I was expecting something like “Disable/Enable HTML messages” under the settings, but instead I found only “default html settings”. So I guess, indirectly, if you select “clear formatting” there, then you will get plain/text encoding for your outgoing emails.
Disabling html is important for mailing lists. Some of them span over millions of people with all sort of devices to read their email. It’s really annoying to get all that html stuff on a small screen from someone who just wants to ask something like “How do I backslash a single quote”.