If you move your cursor to the right of each mail account a + sign and carat should appear. If you click on the + you can add available mailboxes. If you click on the carat it will change direct Click on it so the carat shows down ⌵.
If you still need help post a screenshot of what your Mail app shows in the left side.
To take screenshot
Take a screenshot on your Mac - Apple Support
To add it to a reply
Click on the add image icon right below the test entry box
add Add image image
After clicking on the icon you are present with a Finder like windows. maneuver to find the image file you want to post and click on it and select Open in bottom right or double click on the image file to add the image in the text reply box.