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An email header is the metadata section attached to every email message that contains essential routing and authentication information. Headers include sender and recipient addresses, timestamps, subject lines, and a detailed record of the servers the message passed through during delivery. This technical data enables email servers to properly route messages, verify sender authenticity, and help security systems detect spoofing or tampering attempts.
Email headers are crucial for both security and deliverability. For security professionals, headers reveal the true origin of an email, exposing potential phishing attempts even when the visible 'From' field appears legitimate. By examining the Received chain and authentication results, you can verify whether an email actually came from the claimed sender. From a deliverability perspective, headers contain the authentication signals that inbox providers use to decide whether to accept, quarantine, or reject incoming mail. Emails with proper authentication headers showing SPF, DKIM, and DMARC passes are far more likely to reach the inbox than those failing these checks. Headers also provide essential debugging information when emails go missing or land in spam. Technical support teams rely on header analysis to diagnose delivery failures, identify blacklisted IPs, and resolve routing issues.
Every email consists of two parts: the header and the body. While the body contains your actual message, the header holds the technical information that makes email delivery possible. When you send an email, your mail server adds initial header fields like From, To, Date, and Subject. As the message travels across the internet, each mail server it passes through adds a 'Received' header entry, creating a chronological trail of the email's journey. These entries are stacked in reverse order, so the most recent server appears at the top. This chain helps troubleshoot delivery issues and detect suspicious routing patterns. Authentication headers like Authentication-Results, DKIM-Signature, and Received-SPF are added by receiving servers after checking SPF, DKIM, and DMARC records. These headers indicate whether the email passed or failed various authentication checks, helping spam filters and recipients assess message legitimacy.
In Gmail, open the email and click the three-dot menu, then select 'Show original.' In Outlook, open the message, go to File > Properties, and view the Internet headers section. Most email clients have a similar 'view source' or 'show original' option in the message menu.
Focus on Authentication-Results (shows SPF/DKIM/DMARC pass or fail), Received headers (reveal the actual path and origin), Return-Path (envelope sender address), and DKIM-Signature (cryptographic verification). Discrepancies between these and the visible From address often indicate spoofing.
Some headers like From and Subject can be easily forged by senders. However, Received headers added by each server in the chain are more trustworthy, and authentication headers like DKIM-Signature use cryptography that attackers cannot fake without access to the domain's private key.
Each server that processes the email adds its own headers, and emails may pass through spam filters, security gateways, and multiple mail servers before reaching you. Marketing emails sent through ESPs often have additional tracking and authentication headers, resulting in lengthy header sections.
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