There should be no “email mode” config setting (required for A simple UI with no "bullshit"! Email compatibility, which is a must, rather sooner than later, makes such a config option obsolete. 
The DC text editor interface may, for example, just show the text that will go into the subject of a composed classic email message with a soft shaded background, ending with the shaded standard subject separator “-” in grey. A simple manual carriage return (linefeed) could then serve to manually end the subject and start the body.
(Subject and Group names · Issue #128 · deltachat/deltachat-core · GitHub)
And assuming the defaults from the minimal-set of 1 preset selection (+4 hidden options) at: Use-cases, chat rules and configuration options, everything (chat and email) can work out-of-the-box, non-obtrusive, easily discoverable by the user, and interacting seamlessly.