In order to copy an element to the Clipboard, click the Copy to clipboard -icon in the Tree-view.
In order to paste the copied element into the email, drag the Clipboard-floaty from the top of the Tree-view. Drop it into the Tree-view where you want to paste the element.
You can copy an element in one tab and paste it in another.
Copied elements are remembered across page navigation. You can copy an element, open another email, and paste it there.