Email Marketing & Automation

How Does Email Work? Headers, Routing, and Technology

An email’s journey from the sender or their email service provider (ESP) to the recipient is a complex process involving multiple steps and checkpoints. This journey can end with the email being successfully delivered to the recipient’s inbox, diverting to the junk folder, or bouncing back to the sender temporarily (soft bounce) or permanently (hard bounce). Here’s a step-by-step breakdown of this journey:

  1. Email Composition and Sending: The sender composes an email using an email client or provider. Upon hitting send or scheduling of the email, it is transmitted to the sender’s SMTP (Simple Mail Transfer Protocol) server.
  2. Email Transfer to Recipient’s Server: The SMTP server looks up the recipient’s domain’s MX (Mail Exchange) record in DNS to find the destination server. The email is then relayed from the sender’s SMTP server to the recipient’s SMTP server.
  3. Initial Reception: The recipient’s SMTP server receives the email and conducts preliminary checks, such as verifying if the recipient’s email address exists.
  4. Spam and Security Filters: The email undergoes various spam and security checks.
    • Protocols like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) are used to verify the email’s authenticity.
    • The sender’s IP address may be reviewed for its sending reputation, including via DNSBL lists or an internal email security platform.
    • Content and attachment scanning may also be executed to ensure no malware or spam-like characteristics are present.
  5. Determination of Email Placement: The email is either rejected or routed to the recipient’s inbox or junk/spam folder based on these checks. Factors influencing this include the email’s content, whether the sender is whitelisted as a contact, the sender’s reputation, the recipient’s personal filter settings, and the results of the SPF, DKIM, and DMARC checks.
    • Authentication Checks: Emails failing SPF, DKIM, or DMARC checks are more likely to be flagged as spam.
    • Content Analysis: Emails with suspicious links, keywords, or attachments may be routed to the junk folder.
    • User Interaction: User actions like marking emails as spam or not can influence future email routing.
    • Reputation Scoring: The sender’s reputation, based on past behaviors, affects email delivery.
    • Feedback Loops: Senders enrolled in feedback loops receive information about their emails marked as spam, aiding in adjusting their practices.
  6. Email Delivery: Inbox or Junk Folder: If the email passes all checks, it lands in the recipient’s inbox. If flagged by spam filters or due to policy violations, it is directed to the junk/spam folder.
  7. Notification to Sender: The sender is (normally) sent a delivery code informing them of whether the email is accepted or rejected. Additional response data may be sent to feedback email addresses. This feedback allows senders, especially marketers and organizations, to update their email lists and strategies.

Throughout this journey, the email’s status can be affected by various factors, including network issues, server downtimes, the recipient’s email settings, and the email’s content and structure. Understanding this process is crucial for troubleshooting delivery issues and optimizing email communication strategies.

Email Headers and Email Routing: A Comprehensive Guide

Email headers and routing mechanisms like SPF, DKIM, DMARC, and BIMI are crucial for authenticating your email and ensuring secure and accurate delivery of emails. This section provides an in-depth look at the common functionalities within email headers and explains how emails are routed to the inbox or junk folder.

Common Functionalities in Email Headers

  • From: Indicates the email address of the sender.
  • To: Shows the recipient’s email address.
  • Subject: The subject line of the email.
Subject: Meeting Schedule Update
  • Date: The date and time the email was sent in accordance with the date and time specifications outlined in the Internet Message Format standard, as defined in RFC 5322 (which supersedes RFC 2822 and RFC 822).
Date: Tue, 5 Dec 2023 09:30:00 +0000
  1. MIME-Version: Specifies the MIME version used. Despite various enhancements and extensions to MIME over the years, the version number has remained at 1.0, indicating general adherence to the original MIME standard.
MIME-Version: 1.0
  • Content-Type: Describes the format of the email (e.g., text/plain, text/html).
Content-Type: text/plain; charset="UTF-8"
  • Message-ID: A unique identifier for the email.
Message-ID: <>
  • Received: Tracks each server’s involvement in routing the email.
Received: from [] ( [])
  • Return-Path: Indicates where bounce messages are sent.
Return-Path: <>
  • Reply-To: An alternate email address to reply to.
  • X-Mailer: Shows the email client or software used to send the email.
X-Mailer: MailClient v1.0
  • X-Priority: Indicates the email’s priority (urgent, normal, low).
X-Priority: 1 (High)
  • Feedback Loops (FBLs): Systems that inform senders when recipients mark their emails as spam.
X-Feedback-ID: campaign123:server456
  • List-Unsubscribe Header: Includes a link or email address for easy unsubscription.
List-Unsubscribe: <>, <>
  • X-Headers: Custom headers for additional information or tracking.
X-Campaign-ID: 78910
  • Authentication-Results Header: Contains results of authentication checks like SPF, DKIM, DMARC. If the email passes DMARC authentication, the mail system looks up the BIMI DNS record for the sender’s domain.
Authentication-Results: spf=pass (sender IP is 123.456.78.90)
  • Auto-Submitted Header: Indicates if an email was auto-generated or auto-forwarded.
Auto-Submitted: auto-generated
  • Precedence Header: Helps in managing replies and automated responses.
Precedence: bulk
  • Received-SPF Header: Shows results of the SPF verification that validates the sender’s IP address. It verifies that the domain’s DNS records authorize the sending server.
Received-SPF: Pass (domain of designates 123.456.78.90 as permitted sender)
  • Content-Transfer-Encoding Header: Specifies the encoding for safe email content transport.
Content-Transfer-Encoding: base64

Creating a full example of an HTML email with all the headers we discussed requires combining the MIME (Multipurpose Internet Mail Extensions) standard for formatting the email body and incorporating various header fields. Below is a simplified example to illustrate how it might look:

Subject: Meeting Schedule Update
Date: Tue, 5 Dec 2023 09:30:00 +0000
MIME-Version: 1.0
Content-Type: text/html; charset="UTF-8"
Message-ID: <>
Received: from [] ( [])
Return-Path: <>
X-Mailer: MailClient v1.0
X-Priority: 1 (High)
X-Feedback-ID: campaign123:server456
List-Unsubscribe: <>, <>
X-Campaign-ID: 78910
Authentication-Results: spf=pass (sender IP is 123.456.78.90)
Auto-Submitted: auto-generated
Precedence: bulk
Received-SPF: Pass (domain of designates 123.456.78.90 as permitted sender)
Content-Transfer-Encoding: base64

<!DOCTYPE html>
<title>Meeting Schedule Update</title>
<h1>Meeting Rescheduled</h1>
<p>Dear Team,</p>
<p>Please note that the weekly meeting has been rescheduled to <strong>Thursday, 7th December 2023, at 10:00 AM</strong>.</p>
<p>Best Regards,<br>
The Management Team</p>

This example illustrates an HTML email with various headers:

  • From, To, Subject, and Date headers provide basic email information.
  • MIME-Version and Content-Type specify that this is an HTML email using UTF-8 charset.
  • Message-ID gives a unique identifier to the email.
  • Received and Return-Path are used for tracking and handling bounces.
  • Reply-To, X-Mailer, and X-Priority offer additional information about reply mechanism, email client used, and priority of the email.
  • X-Feedback-ID, List-Unsubscribe, and X-Campaign-ID are particularly relevant for marketing and automated emails.
  • Authentication-Results and Received-SPF provide information on the email’s authentication status.
  • Auto-Submitted and Precedence headers can influence how auto-responders treat the email.
  • Content-Transfer-Encoding indicates how the body of the email is encoded.

This is a simplified example for illustrative purposes. In a real-world scenario, the formatting and composition of email headers can be more complex, especially in automated or marketing emails. Each header contributes to email systems’ smooth functioning, enhancing digital communication security, manageability, and user experience.

How To View Email Headers

Viewing email headers in various email clients like macOS Mail, Outlook, Gmail, and others can be a valuable tool for troubleshooting, understanding email delivery paths, or verifying email authenticity. Here’s how to view email headers in some of the major email clients:

macOS Mail

  1. Open the Email: Click to open the email whose headers you want to view.
  2. View Headers: Click View in the menu bar, select Message, and choose Raw Source or All Headers from the submenu.

Microsoft Outlook (Desktop Application)

  1. Open the Email: Double-click the email to open it in a new window.
  2. File Tab: Click on the File tab in the ribbon.
  3. Properties: Under the Info menu, click on Properties. The Internet headers section at the bottom of the Properties window shows the full email headers.

Outlook for Web (

  1. Open the Email: Click to open the email.
  2. More Actions: Click on the three dots (More actions) in the top-right corner of the email.
  3. View Message Source: Select View message source from the dropdown menu.


  1. Open the Email: Click to open the email.
  2. More Options: Click on the three dots (More options) in the top-right corner of the email, next to the reply button.
  3. Show Original: Choose Show original from the dropdown menu. This opens a new tab/window with the full email headers and raw source.
View email headers in Gmail menu

Yahoo Mail

  1. Open the Email: Click to open the email.
  2. More Options: Click on the More icon (usually three dots or a hamburger icon) at the top of the email.
  3. View Raw Message: Select View Raw Message or View Full Header from the dropdown menu.


  1. Open the Email: Click to open the email.
  2. View Headers: Click on View in the menu bar, then choose Headers, and select All to display full headers.

The process may vary slightly for other email clients, but generally, you will find the option to view headers in the menu options associated with the individual email message. Often it’s under View, More Options, or a similar menu item.

Google Admin Toolbox: Messageheader

Analyzing the text in a message header is not easy, but Google has a great tool where you can just paste raw emails with headers, and it will summarize all the header details and email routing.

email message header 3Google Admin Toolbox: View Email Message Header
Slide to view the raw source, then the summary

Analyze an Email Message Header

Understanding these elements of email headers and routing protocols is vital for every business.

Adam Small

Adam Small is the CEO of AgentSauce, a full-featured, automated real estate marketing platform integrated with direct mail, email, SMS, mobile apps, social media, CRM, and MLS.

Related Articles

Back to top button