View long email headers in Mail on iCloud.com
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.
In Mail on iCloud.com, 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 by choosing Hide Long Headers in the Action pop-up menu .