The setting is useful, thank you.

My preference is only related to the fact that currently the just sent note is hidden, and I cannot post another note in the meantime.

Probably a good general solution could be allowing undo in place for 3 seconds (enought to spot basic errors) showing the note, then show again the compose area and let the user to undo until X seconds accessing "My notes", where he can find a button for every "sending" note.

In the sidebar we can show if there are email being sent, clicking the component would bring to "My notes".