iCloud

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 in the toolbar, then choose Show Long Headers.
After you choose to show long headers, they continue to appear until you turn them off. Click , then choose Hide Long Headers.

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iCloud User Guide

View long email headers in Mail on iCloud.com