# Email Headers and Message Format > Message headers, MIME structure, and basic formatting rules help inboxes parse 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 free tool, pair the 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 parse and trust an email. Email is not only the visible design. Every message has headers and a body. Headers carry information such as From, To, Subject, Date, Message-ID, Reply-To, List-Unsubscribe, DKIM signatures, and routing details. Inbox providers expect messages to follow internet message format rules. If headers are missing, duplicated in the wrong way, too large, or misleading, delivery can suffer even when the design looks fine. 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 exist. The Reply-To should point to a real inbox when replies matter. MIME is 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, where supported by the sending system, a plain-text version. HTML email should be simple and resilient. Some email clients block images, ignore certain CSS, clip very large messages, or render the same layout differently. That is why testing on desktop and mobile matters. Message format is not a substitute for permission or authentication. It is a baseline. 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 recognizable From name and From email. 2. Write a subject line that matches the actual message. 3. Use the [Email Editor](https://mailrith.com/docs/email-editor.md) to build a clean body instead of pasting complicated HTML from unknown sources. 4. Keep images useful and avoid putting important text only inside an image. 5. Send a test email and open it in at least one desktop inbox and one mobile inbox. 6. For deliverability troubleshooting, inspect the original headers and check From, Reply-To, Message-ID, SPF, DKIM, DMARC, and List-Unsubscribe results. 7. If an email delivery service reports a message-format error, fix the header, MIME, HTML, or size issue before retrying a large send. - The visible email is only one part of the message. - Headers help inboxes understand sender identity, routing, authentication, and unsubscribe behavior. - Message-ID, Date, From, To, and Subject should be valid and not duplicated incorrectly. - Subject lines and display names should not mislead people. - Large or messy HTML can display poorly and may trigger clipping 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 A header analyzer says the pasted text does not contain original email headers. 1. Open the delivered test email in the receiving inbox. 2. In Gmail, open the three-dot menu and choose `Show original`. 3. In Outlook or Apple Mail, look for message source, view source, or internet headers. 4. Copy the header block that includes From, Return-Path, Received, Authentication-Results, and DKIM-Signature if present. 5. Paste the headers into the analyzer again. 6. Remove private body content if you do not need it checked. ### From Domain Not Found A header analyzer cannot find the visible From domain in the pasted headers. 1. Make sure the pasted content includes a line that starts with `From:`. 2. Send a fresh test email if the original message source is incomplete. 3. Check that the From address is a normal email address such as `name@example.com`. 4. Fix the sender identity in the sending tool or email delivery service if the From header is malformed. 5. Send another test and analyze the new headers. ### Authentication-Results Missing A header analyzer cannot find the receiver's SPF, DKIM, and DMARC verdicts. 1. Copy headers from the inbox that received the email, not 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 and compare SPF, DKIM, DMARC, From, Return-Path, and DKIM `d=` domains. ### Email Too Large or Clipped An HTML size checker says the email is near or above a common clipping threshold. 1. Remove unused layout sections, comments, and pasted template code. 2. Shorten repeated content and move long supporting detail to a landing page. 3. Avoid embedding base64 images in the HTML. 4. Keep only the images that help the reader understand or act. 5. Send a test email to Gmail and confirm the full message, footer, and unsubscribe link are visible. ### Script Tag Found An HTML checker found a script tag inside the email content. 1. Remove every `