[lxgr]

"Plain text mode", hiding in the "kebab menu" (i.e. the three vertically stacked dots) on the bottom of the message composition window. It even seems to stay on as a default once activated!

Very useful for the few times I actually need to send email to mailing lists with strong opinions about newfangled MIME multipart messages :)

[Suzuran]

Yes, that's very useful, thanks for pointing that out.