By default, only messages sent to "Everyone" or "Students" in the recipient list automatically appear on the class email archive page. There might be situations where you send an email to a smaller group of people but you want that message to be available to everyone on the class email archive page.
If you check the Display on the class email archive page checkbox (under "Options"), the message will appear everyone on the class email archive page.
Be sure that you do not select this option when you are discussing individual student grades or issues specific to individual students. You do not want those kinds of messages appearing to everyone in the class on the class email archive page.