Using New Outlook on Windows for professional communication and productivity
You are clicking the 3 dots in the message section. Above that, and below "File", "Message", etc., you should find a Ribbon that has icons for trash, move, etc. At the end of that Ribbon line, you should see 3 more dots. This will lead to a drop-down list where you select "Actions", then "Edit Message".
This will allow you to make notes (and edits) directly in the email message, but not as you described in excel.