iCloud: See more details about an email
By default, each email you receive displays the email’s sender, recipients, and date. If you want to see more details about a message—such as an email’s return and delivery paths, “Reply-to” field, and content type—you can display its long header.
View long message headers
Double-click a message to open it in a new window.
Click the Action pop-up menu in the toolbar, then choose Show Long Headers.