# Email Headers and Message Format



> Message headers, MIME structure, and basic formatting rules help inboxes read, display, and trust an email.



- Human page: https://mailrith.com/guides/email-headers-and-message-format

- Markdown page: https://mailrith.com/guides/email-headers-and-message-format.md

- Category: Authentication and Deliverability

- Reading time: 7 min read

- Related keywords: Email Headers and Message Format, Email Headers and Message Format guide, Authentication and Deliverability, Authentication and Deliverability guide, email sending guide, email marketing guide, email deliverability guide, Email Editor, RFC 5322 Internet Message Format, One-Click Unsubscribe and Opt-Outs



## AI Agent Notes

- Use this page as plain-language guidance for the specific email sending issue named in the title.

- Preserve the distinction between Mailrith, an email delivery service, DNS, and inbox providers when explaining fixes.

- When a user is running a Mailrith free tool, pair that specific free-tool result with the relevant issue or step section from this guide.



### Email Headers and Message Format

Message headers, MIME structure, and basic formatting rules help inboxes read, display, and trust an email.

An email is more than the design people see. Every message has headers and a body. Headers include information such as From, To, Subject, Date, Message-ID, Reply-To, List-Unsubscribe, DKIM signatures, and routing details.

Inbox providers expect each message to follow internet message format rules. Delivery can suffer if headers are missing, duplicated incorrectly, too large, or misleading, even when the email design looks correct.

Important headers should be clear and consistent. The From header should identify the sender. The Subject should describe the message honestly. The Date and Message-ID should be present. The Reply-To should use a real inbox when replies matter.

MIME defines the structure that lets one email contain different parts, such as HTML, plain text, images, attachments, and tracking elements. A healthy marketing email usually has a clear HTML body and, when the sending system supports it, a plain-text version.

HTML email should be simple and reliable. Some email clients block images, ignore certain CSS, clip very large messages, or display the same layout differently. Test each email on desktop and mobile before sending.

Message format does not replace permission or authentication. It is a baseline requirement. A well-formatted unwanted email can still go to spam, but a badly formatted wanted email may be rejected or displayed poorly.

1. Use a From name and From email that recipients can recognize.
2. Write a subject line that accurately describes the message content.
3. Use the [Email Editor](https://mailrith.com/docs/email-editor.md) to build a clean email body instead of pasting complicated HTML from unknown sources.
4. Use images only when they help the message, and do not put important text only inside an image.
5. Send a test email. Open the test email in at least one desktop inbox and one mobile inbox.
6. For deliverability troubleshooting, inspect the original headers. Check that From, Reply-To, Message-ID, SPF, DKIM, DMARC, and List-Unsubscribe results are present and match the expected sender information.
7. If an email delivery service reports a message-format error, fix the reported header, MIME, HTML, or size problem before you retry a large send.

- The visible email content is only one part of the full message.
- Headers help inboxes understand sender identity, routing, authentication, and unsubscribe behavior.
- Message-ID, Date, From, To, and Subject should be valid and should not be duplicated incorrectly.
- Subject lines and display names should not mislead recipients.
- Large or messy HTML can display poorly and may be clipped in some inboxes.
- Use [One-Click Unsubscribe and Opt-Outs](https://mailrith.com/guides/one-click-unsubscribe.md) to understand unsubscribe headers.
- Use [From, Reply-To, and Return-Path](https://mailrith.com/guides/from-reply-to-and-return-path.md) to understand sender-related headers.

## Fix Common Issues
### Original Headers Missing

An email header analyzer says the pasted text does not contain original email headers.

1. Open the delivered test email in the inbox that received it.
2. In Gmail, open the three-dot menu and choose `Show original`.
3. In Outlook or Apple Mail, look for the message source, view source, or internet headers option.
4. Copy the header block. Make sure it includes From, Return-Path, Received, Authentication-Results, and DKIM-Signature if those headers are present.
5. Paste the copied headers into the analyzer again.
6. Remove private body content before analysis if you do not need the analyzer to check the body.

### From Domain Not Found

An email header analyzer cannot find the visible From domain in the pasted headers.

1. Check the pasted headers for a line that starts with `From:`.
2. Send a new test email if the original message source does not include the full headers.
3. Check that the From address uses a normal email address format, such as `name@example.com`.
4. If the From header is malformed, fix the sender identity in the sending tool or email delivery service.
5. Send another test email and analyze the new headers.

### Authentication-Results Missing

An email header analyzer cannot find the receiving inbox's SPF, DKIM, and DMARC verdicts.

1. Copy headers from the inbox that received the email. Do not copy headers from the draft or sending preview.
2. Look for a header named `Authentication-Results`.
3. If the receiving inbox does not add that header, send the test to Gmail, Outlook, or another inbox that shows original authentication results.
4. Analyze the new headers. Compare SPF, DKIM, DMARC, From, Return-Path, and DKIM `d=` domains against the expected sending domains.

### Email Too Large or Clipped

An HTML email size checker says the email is near or above a common clipping threshold.

1. Remove unused layout sections, comments, and pasted template code from the HTML.
2. Shorten repeated content and move long supporting information to a landing page.
3. Do not embed base64 images in the HTML.
4. Keep only the images that help the reader understand the message or take action.
5. Send a test email to Gmail. Confirm that the full message, footer, and unsubscribe link are visible.

### Script Tag Found

An HTML email size checker found a script tag inside the email content.

1. Remove every `<script>` tag from the email HTML.
2. Do not rely on JavaScript for layout, tracking, buttons, forms, countdowns, or personalization inside email.
3. Use email-safe HTML and normal links instead.
4. Send a test email and confirm that the email content still works without scripts.

### Form Tag Found

An HTML email size checker found a form inside the email content.

1. Remove the embedded form from the email.
2. Create a hosted form or landing page instead.
3. Link to the hosted form or landing page from one clear button or text link.
4. Send a test email and click the link. Confirm that the form opens correctly.

### Base64 Image Found

An HTML email size checker found an embedded base64 image in the email content.

1. Upload the image to a reliable HTTPS image host or use your email editor's image upload.
2. Replace the embedded base64 data with a normal HTTPS image URL.
3. Add useful alt text if the image supports the message.
4. Run the HTML email size checker again and send a test email.

### Image-Heavy Email

An HTML email size checker found many images in the email.

1. Remove decorative images that do not help the reader understand the message.
2. Keep important text as real text, not only inside images.
3. Make sure the main call-to-action is visible even when images are blocked.
4. Send a mobile test email. Confirm that the message is easy to read on a mobile screen.

> If the email delivery service says the message is malformed, do not keep resending the same message. Fix the format problem first, then send another test.

Related resources:
- [Email Editor](https://mailrith.com/docs/email-editor.md): Use Mailrith's editor to build and preview email content.
- [RFC 5322 Internet Message Format](https://www.rfc-editor.org/rfc/rfc5322): Read the technical standard for internet email message format.
- [One-Click Unsubscribe and Opt-Outs](https://mailrith.com/guides/one-click-unsubscribe.md): Understand List-Unsubscribe headers and body unsubscribe links.



## Related Guides

- [Sender Domains and Email Authentication](https://mailrith.com/guides/sender-domains-and-authentication.md): Your sender domain is what inbox providers learn to trust. Authentication proves that your email delivery service is allowed to send email for your domain.

- [From, Reply-To, and Return-Path](https://mailrith.com/guides/from-reply-to-and-return-path.md): An email can include several sender-related addresses. Each address has a separate role for delivery, authentication, or replies.

- [DNS, PTR, and Reverse DNS](https://mailrith.com/guides/dns-and-reverse-dns.md): DNS records identify your domain. Reverse DNS helps inbox providers check that a sending IP address has a valid hostname.