Back to Glossary
Email Structure

Email Header

Metadata at the top of an email containing routing information, authentication results, and sender details.

Definition

Email headers are lines of metadata at the beginning of an email message containing technical information about its journey. Headers include sender and recipient addresses, subject line, timestamps, server hops, authentication results (SPF, DKIM, DMARC), and message IDs. While mostly hidden from recipients, headers are essential for deliverability troubleshooting.

Why It Matters

Headers contain crucial information for diagnosing delivery problems. They show authentication pass/fail results, reveal the sending server path, and help identify why emails might be flagged as spam. Understanding headers helps troubleshoot deliverability issues and verify proper authentication setup.

How It Works

Each server handling an email adds header lines. Headers are processed bottom-to-top chronologically (newest at top). Common headers include From, To, Subject, Date, Message-ID, Received (showing server path), and Authentication-Results. Most email clients hide headers but allow viewing in message options.

Example

Example email headers (simplified):

Received: from mail.sender.com (192.168.1.1) by mx.gmail.com; Mon, 15 Jan 2024 10:30:00 -0800 Authentication-Results: mx.google.com; dkim=pass header.d=sender.com; spf=pass smtp.mailfrom=sender.com; dmarc=pass policy.dmarc=sender.com From: [email protected] To: [email protected] Subject: Your Weekly Update Date: Mon, 15 Jan 2024 10:29:55 -0800 Message-ID: <[email protected]>

Best Practices

  • 1Learn to read headers for troubleshooting deliverability
  • 2Check Authentication-Results to verify SPF, DKIM, DMARC pass
  • 3Use header analyzers (like MXToolbox) to interpret complex headers
  • 4Monitor for unusual routing that might indicate problems
  • 5Ensure your From header matches your authenticated domain

Frequently Asked Questions

In Gmail: Open the email, click the three dots, select 'Show original'. In Outlook: Open the email, click File > Properties. Most email clients have similar options. The full headers appear as plain text.

Focus on: Authentication-Results (should show pass for SPF, DKIM, DMARC), Received headers (for routing issues), and X-Spam headers (if present, shows spam scoring). These reveal why emails succeed or fail.