Update 2019-05-14: not anymore! We now make sure that when you copy and paste into the email editor, your text will be pasted as plain text. See Why can’t I copy and paste with formatting into an email?
When you paste text directly into the editor, formatted text is not excluded, and therefore, the result is different styles, fonts and font sizes. Use the buttons ”Paste Plain Text” or ”Paste from Word”, then the formatting will be removed and the result will look just the way it should.