The copy-paste function can have unexpected results when copying text from a Word document or other text editing applications and trying to paste it into the email editor. It is recommended that you use the Notepad app for Microsoft or the TextEdit app in Mac/Apple. You can copy and paste the text from your original document into the Notepad/TextEdit app, which will transform the text into plain text so that no formatting is hidden in the text.
Once your text is in the Notepad/TextEdit app, you can copy from there highlighting the text and using Edit>Copy or Ctrl+C (press the ctrl key and the letter "C" at the same time) to copy.
Double click on the text box where you would like to paste the text. Then highlight the area where the text will be pasted and use Ctrl+V (press the ctrl key and the letter "V" at the same time) to paste.