Email Headers ​

Email headers are the hidden metadata that travel with every message you send or receive. While most users only see the subject line and body content, these headers contain vital information about the message's journey, authentication status, and technical properties. Think of them as a message's digital passport with stamps from every server it passed through.

What are Email Headers? ​

Email headers are structured metadata fields placed at the beginning of an email message that contain information about the sender, recipient, routing path, and authentication results. They're organized as key-value pairs, with each header field typically appearing on its own line.

The most common headers include:

• From: The sender's display name and email address
• To: The recipient's email address
• Subject: The email's subject line
• Date: When the message was sent
• Message-ID: A unique identifier for the message
• Received: Tracking information showing the message's path across servers
• Authentication-Results: Results of SPF, DKIM, and DMARC checks

Beyond these standard fields, emails typically contain numerous technical headers related to content type, encoding, priority, and spam assessment. While largely invisible to everyday users, these headers are essential infrastructure for how email functions on the modern internet.

Here's an example of what basic email headers look like:

From: "John Smith" <john@example.com>
To: recipient@domain.com
Subject: Meeting Tomorrow
Date: Mon, 6 Jun 2025 10:30:45 -0700
Message-ID: <a1b2c3d4e5@mail.example.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="UTF-8"

How Headers Are Created ? ​

Email headers operate as an information layer that guides how your message is processed from creation to delivery. They're progressively built as your message traverses the internet:

1. Initial Headers: Your email client creates the basic headers (From, To, Subject, Date)
2. Sending Server: Your outgoing SMTP server adds headers about the sending environment and authentication
3. Transit: Each intermediary server adds a new Received header, creating a trail of the email's journey
4. Receiving Server: The destination mail server adds authentication results and spam analysis headers
5. Email Client: The recipient's mail client uses these headers to display the message appropriately

Authentication Headers and Security ​

The Received headers create a chronological record of the email's path, with the most recent server at the top. Reading them from bottom to top shows the complete journey from sender to recipient.

Here's an example of the authentication headers added during delivery:

Authentication-Results: mx.recipient.com;
       dkim=pass (signature verified) header.d=example.com;
       spf=pass (sender IP is authorized) smtp.mailfrom=john@example.com;
       dmarc=pass (policy=reject) header.from=example.com
Received-SPF: pass (mx.recipient.com: domain of john@example.com designates 192.0.2.1 as permitted sender) client-ip=192.0.2.1;
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com; s=20210601;
 h=from:to:subject:date:message-id;
 bh=A1B2C3D4E5F6G7H8I9J0K1L2M3N4O5P6Q7R8S9T0U1V2W3X4Y5Z6=;
 b=A1B2C3D4E5F6G7H8I9J0K1L2M3N4O5P6Q7R8S9T0U1V2W3X4Y5Z6A1B2C3D4E5F6G7H8
   I9J0K1L2M3N4O5P6Q7R8S9T0U1V2W3X4Y5Z6A1B2C3D4E5F6G7H8I9J0K1L2M3N4O5P6

Let's break this down:

Authentication-Results: The receiving server's summary of all authentication checks

• dkim=pass: The digital signature matched, confirming the message hasn't been altered
• spf=pass: The sending server is authorized to send email for this domain
• dmarc=pass: The message passed both DKIM and SPF checks according to the domain's policy

Received-SPF: Detailed results of the SPF check

• pass: The email came from an authorized IP address
• client-ip=192.0.2.1: The specific IP address that sent the message

DKIM-Signature: The digital signature attached to verify the message

• v=1: The DKIM version
• a=rsa-sha256: The encryption algorithm used
• d=example.com: The domain claiming responsibility for the message
• s=20210601: The "selector" - pointer to the correct public key
• h=from:to:subject...: List of headers included in the signature
• bh=...: A hash of the email body (ensures content hasn't changed)
• b=...: The actual encrypted signature (the digital seal)

This layered approach creates a verifiable chain of custody for each email. When properly implemented, headers make it nearly impossible for senders to falsify their identity without being detected by modern authentication systems.

Why are Email Headers Important? ​

Email headers are crucial for security and deliverability as they provide the technical foundation for all major email authentication protocols. They help mailbox providers determine message legitimacy, enable advanced features like conversation threading, and serve as invaluable troubleshooting tools when delivery issues arise. Without properly configured headers, messages often land in spam folders or get rejected entirely. For businesses, optimizing these headers is essential for maintaining high inbox placement rates, while for technical teams, headers provide the diagnostic information needed to resolve delivery problems and improve email performance. Even as email has evolved over decades, headers remain the consistent technical backbone holding the entire system together.

Frequently Asked Questions About Email Headers ​

How can I view the full headers of an email? ​

Most email clients hide headers by default but provide ways to view them. In Gmail, open the email and click the three dots menu, then "Show original." In Outlook, open the message, click "File" and then "Properties" to see the internet headers. Apple Mail users can select "View" and then "Message" followed by "All Headers."

Can I modify email headers to improve deliverability? ​

Some headers like Message-ID, DKIM-Signature, and Return-Path should be properly configured in your sending infrastructure to improve deliverability. However, tampering with headers after sending is impossible without breaking authentication signatures. Instead, focus on proper SPF, DKIM, and DMARC setup.

Do email headers affect privacy? ​

Yes, headers contain information that can reveal your IP address, email client, and sometimes geographic location. When forwarding sensitive emails, consider using "forward as attachment" options to avoid exposing your original headers or use specialized privacy-focused email services that minimize header information.

Related Content ​

• SMTP (Simple Mail Transfer Protocol)
• SPF (Sender Policy Framework)
• DKIM (DomainKeys Identified Mail)
• DMARC (Domain-based Message Authentication, Reporting & Conformance)
• Return Path
• Email Authentication