Reading Email Headers to Diagnose Deliverability and Spam/Junk Issues - Systron Micronix Corporation

Reading Email Headers to Diagnose Deliverability and Spam/Junk Issues Print

email headers, Email, spam, Junk Folder

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Email headers reveal the path, authentication, and filtering decisions applied to a message. Reading them correctly helps you pinpoint why emails land in junk, get delayed, or fail authentication. This guide explains where to find headers, how to interpret key fields, and a practical troubleshooting workflow.

What an email header contains

Routing: A chain of Received: lines showing each server hop with timestamps.
Authentication: Authentication-Results: for SPF, DKIM, DMARC (and often ARC-* for forwarding).
Identity: From:, Return-Path:, Reply-To:, Message-ID:, Sender:.
Filtering: Provider-specific spam verdicts (e.g., X-Spam-Status:, ARC-Seal:, X-MS-Exchange-*, Received-SPF:).
List/meta: List-Unsubscribe:, MIME details, Content-Type:, and user agent.

Goal: Confirm whether authentication aligned, identify which server flagged spam, and correlate timing or content changes with deliverability outcomes.

How to view full headers in common email clients

Gmail (web)

Open the message. Click the three dots (More) near the top right.
Choose “Show original”. A new tab opens with full headers and Gmail’s SPF/DKIM/DMARC summary.
Copy headers. Click “Copy to clipboard” for raw headers.

Gmail - Show original location in message options

Gmail: Use “Show original” to access full headers and authentication summary.

Outlook (desktop)

Open the message. Go to File → Properties.
Find Internet headers. Copy the contents of the “Internet headers” box.

Outlook - Properties dialog showing Internet headers

Outlook: Internet headers are in File → Properties. Outlook - showing Internet headers

Outlook: Internet headers are in File → Headers.

Apple Mail (macOS)

Open the message. Choose View → Message → All Headers or View → Message → Raw Source.
Copy raw source. Raw source includes full headers and MIME body.

Apple Mail: View all headers or raw source for complete details. Apple Mail - View All Headers option

Apple Mail: View all headers or raw source for complete details.

Thunderbird

Open the message. Press Ctrl+U (Windows/Linux) or Cmd+U (macOS).
Select the message and go to View > Message Source.
Alternatively, highlight the email in your inbox, then go to Menu > View > Message Source.
To view an email’s headers, select it and go to Menu > View > Headers > All.
Headers appear. A new window shows full raw headers and body.

Thunderbird: View source shows raw headers quickly.

Key header fields and how to interpret them

Authentication-Results

SPF: Looks at the connecting IP’s authorization for the envelope domain. Common results: pass, fail, softfail, neutral.
DKIM: Verifies the digital signature; results include pass, fail, none, and alignment status.
DMARC: Requires alignment with SPF or DKIM; results include pass or fail, often with policy=none|quarantine|reject.

ARC (Authenticated Received Chain)

ARC-Seal / ARC-Message-Signature: Preserve authentication across forwarders; useful when DMARC fails post-forward.
If forwarding: ARC can explain why a previously authenticated message is still accepted.

Received

Hop chain: Each Received: line shows a server hop and timestamp; read bottom-up (first hop) to top (final hop).
Latency: Large gaps indicate delays; mismatched timezones or clocks can complicate analysis.
IP and HELO: Confirms the sending IP and hostname; mismatches may reduce trust.

Return-Path and From

Return-Path (envelope-from): Used for SPF; may differ from From:.
Alignment: DMARC expects organizational-domain alignment between identity and authentication.
Reply-To: Should be a monitored mailbox; “no-reply” may harm engagement.

Provider spam verdicts

Gmail: Authentication-Results with spf=, dkim=, dmarc=; sometimes X- headers.
Microsoft: Authentication-Results, Received-SPF, various X-MS-Exchange-*, SpamDiagnosticOutput.
Gateways: X-Spam-Status, X-Spam-Flag, X-Spam-Score from SpamAssassin-like systems.

Message-ID and List-Unsubscribe

Message-ID: Unique identifier; malformed or missing can look suspicious.
List-Unsubscribe: Improves trust; may enable one-click unsubscribe parsing by inboxes.

Sample header and interpretation

Delivered-To: user@example.com
Return-Path: <bounces@mailer.yourdomain.com>
Received: from mailer.yourdomain.com (mailer.yourdomain.com. [203.0.113.25])
        by mx.google.com with ESMTPS id abc123
        for <user@example.com>
        (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384) Sat, 10 Dec 2025 10:15:32 -0800 (PST)
Received-SPF: pass (google.com: domain of bounces@mailer.yourdomain.com designates 203.0.113.25 as permitted sender) client-ip=203.0.113.25;
Authentication-Results: mx.google.com;
        spf=pass (google.com: domain of bounces@mailer.yourdomain.com designates 203.0.113.25 as permitted sender) smtp.mailfrom=bounces@mailer.yourdomain.com;
        dkim=pass header.d=yourdomain.com header.s=mail header.b=ABC123;
        dmarc=pass (p=quarantine dis=none) header.from=yourdomain.com
ARC-Seal: i=1; a=rsa-sha256; d=forwarder.example; s=arc; t=1733831732; cv=pass;
From: Systron <news@yourdomain.com>
Reply-To: support@yourdomain.com
Subject: December product update
Message-ID: <20251210-mailer-xyz@yourdomain.com>
List-Unsubscribe: <mailto:unsubscribe@yourdomain.com>, <https://yourdomain.com/u/12345>

SPF pass: The sending IP is authorized for the envelope domain.
DKIM pass: Signature from yourdomain.com validated.
DMARC pass: From domain aligns with either SPF or DKIM (here, DKIM aligns).
ARC present: Indicates forwarding preserved authentication.
Verdict: Healthy authentication; if still junked, investigate content, reputation, or engagement factors.

Common failure patterns and fixes

SPF softfail or too many lookups

Header clue: spf=softfail or Received-SPF: softfail; “exceeds DNS lookups.”
Fix: Consolidate includes; keep under 10 lookups; move to -all after validation; ensure the sending IP is covered.

DKIM none or fail

Header clue: dkim=none (no signature) or dkim=fail (bad key or altered content).
Fix: Enable signing, correct selector, rotate and publish keys, ensure your MTA/ESP applies DKIM to all messages.

DMARC fail (unaligned domains)

Header clue: dmarc=fail, policy=quarantine|reject, header.from domain doesn’t align with SPF/DKIM identity.
Fix: Send from your organizational domain, align SPF (envelope-from) or DKIM (d= domain) with From: domain; adjust p= after validation.

Forwarding breaks DMARC

Header clue: spf=fail after forwarding; ARC present with cv=pass.
Fix: Rely on DKIM alignment (less affected by forwarding); ARC can help receivers trust forwarded mail.

Gateway spam flags

Header clue: X-Spam-Status: Yes, high score; Microsoft SpamDiagnosticOutput indicates policy hit.
Fix: Reduce spam triggers (links, wording, images), ensure HTML and text parts are clean, check attachments and reputation.

Decision tree: diagnose with headers

Start: Open full headers; capture Authentication-Results, Received, and spam verdicts.
Authentication: If SPF/DKIM/DMARC all pass and align, proceed to content/reputation. If any fail, fix those first.
Routing: Check Received chain for delays or unexpected hops; confirm HELO and PTR match your domain.
Identity: Ensure From, Return-Path, and DKIM d= use your organizational domain.
Spam verdicts: Note which system flagged the message; adjust content and sending patterns accordingly.
Re-test: Send to engaged recipients; confirm improved placement; scale gradually.

Screenshot walkthrough: reading headers step by step

1. Gmail “Show original” summary

Gmail Show original - Authentication results for SPF, DKIM, DMARC

Gmail provides a quick summary of SPF/DKIM/DMARC in “Show original”. Look for “PASS” or “FAIL”.

2. Outlook Internet headers

Outlook Internet headers with Received and Authentication-Results lines

Outlook’s Internet headers pane shows Received chain, Authentication-Results, and provider-specific flags.

3. Apple Mail raw source

Apple Mail raw source showing full header metadata

Apple Mail raw source reveals all headers, including ARC and List-Unsubscribe where present.

Tip: Save the header as a text file and annotate key lines (SPF/DKIM/DMARC, Received hops). It speeds up collaboration with IT or your ESP.

Header red flags and quick remediations

SPF softfail/neutral: Add/verify sending IPs or includes; keep lookup count < 10.
DKIM missing: Enable DKIM, correct selector, rotate keys, ensure all streams sign.
DMARC unaligned: Bring From:, SPF envelope, and DKIM signing domain under the same organizational domain.
Broken HELO/PTR: Align HELO with FQDN, ensure reverse DNS (PTR) resolves to your sending hostname.
Gateway spam score high: Remove risky attachments, tidy HTML, reduce image-to-text ratio, and simplify links.

Checklist: what to capture from headers for a deliverability ticket

Full raw headers: Unmodified, pasted in a code block or text file.
Provider summary: SPF/DKIM/DMARC verdicts and policy details.
Received chain: Hops with timestamps; note latency gaps.
Identity alignment: From, Return-Path, DKIM d=, Message-ID domain.
Spam flags: Any X-Spam-*, Received-SPF, Microsoft SpamDiagnosticOutput, ARC results.

Next steps after fixing header-level issues

Content cleanup: Minimize spammy language and excessive formatting; include a plain-text part.
Reputation: Check blocklists, warm new IPs, and stabilize cadence.
Compliance: Visible one-click unsubscribe, physical address, accurate sender identity.
Engagement: Target engaged segments first; trim inactive recipients to bolster positive signals.

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Reading Email Headers to Diagnose Deliverability and Spam/Junk Issues Print

What an email header contains