iCloud: See more details about an email in iCloud Mail
By default, each email you receive displays the email’s sender, recipients, and date. When using Mail on iCloud.com 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.
After you choose to show long headers, they continue to appear until you turn them off.