メールヘッダーアナライザー

What Are Email Headers and What Do They Contain?

Every email message contains two parts: the visible body (the text, images, and HTML you read) and the technical headers that live above the body. Headers record the full routing history of the message, authentication results, spam scores, and metadata like the sender, recipient, subject, and timestamps. Most email clients hide headers from normal view, but they are always present and can be revealed through your mail client's "show original" or "view headers" option.

Analyzing email headers is one of the most reliable ways to diagnose delivery problems, investigate suspected phishing messages, and verify that your authentication setup (SPF, DKIM, DMARC) is working correctly for a real message.

How to Find Email Headers in Different Mail Clients

• Gmail:Open the message, click the three-dot menu (⋮) in the top-right corner, and select "Show original". Copy all content from the popup.
• Outlook (web):Open the message, click the three-dot menu, then View → View message source.
• Apple Mail:Open the message, then View → Message → All Headers, or press Shift + Command + H.
• Thunderbird:Open the message, then View → Headers → All.
• Yahoo Mail:Open the message, click More → View Raw Message.

Copy everything from the first header line (usually Delivered-To: or Return-Path:) down to the blank line before the email body.

Key Headers to Look For

The most important headers for diagnosing delivery issues are:

• Authentication-Results: Shows SPF, DKIM, and DMARC pass/fail results as evaluated by the receiving server. This is the most important header for diagnosing authentication failures.
• Received: One entry per hop in the delivery path. Reading from bottom to top gives chronological order. Each entry usually includes the server hostname, IP address, and timestamp.
• X-Spam-Score / X-Spam-Status: Added by spam filters like SpamAssassin. Shows the numeric score and which rules were triggered.
• DKIM-Signature: The actual DKIM signature attached by the sending server. Contains the selector, domain, and cryptographic signature.
• Return-Path: The envelope sender — the address used for bounce notifications, checked by SPF.
• Message-ID: A unique identifier for the message, useful for tracking it through log files.

What Authentication Results Mean for Deliverability

A message showing spf=pass dkim=pass dmarc=pass in the Authentication-Results header has the best chance of inbox placement. Any failure here should be investigated. An SPF failure often means the sending IP is not listed in your SPF record. A DKIM failure may mean the signature is missing, expired, or the public key in DNS does not match the private key used for signing. A DMARC failure means neither SPF nor DKIM aligned with the From domain.

Header analysis tells you what happened with a specific message. To prevent problems before sending, use email verification on your list and verify your authentication configuration with our SPF, DKIM, and DMARC generator tools. For large lists, bulk verification and the validation API let you integrate checks into your workflow.