--- title: "7 Steps To Verify ARC Seals Using Popular Email Header Analysis Tools | DuoCircle" description: "Learn 7 simple steps to verify ARC seals using popular email header analysis tools to validate authentication chains and improve email deliverability." image: "https://www.duocircle.com/images/og-default.png" canonical: "https://www.duocircle.com/blog/7-steps-verify-arc-seals-popular-email-header-analysis-tools/" --- Quick Answer Verifying ARC seals helps confirm the integrity of forwarded email authentication. Learn the 7 essential steps to verify ARC-Seal, ARC-Message-Signature, and ARC-Authentication-Results using popular email header analysis tools to improve email security and troubleshoot delivery issues. Share [ ](https://www.linkedin.com/sharing/share-offsite/?url=undefined%2Fblog%2F7-steps-verify-arc-seals-popular-email-header-analysis-tools%2F "Share on LinkedIn") [ ](https://twitter.com/intent/tweet?text=7%20Steps%20To%20Verify%20ARC%20Seals%20Using%20Popular%20Email%20Header%20Analysis%20Tools%20&url=undefined%2Fblog%2F7-steps-verify-arc-seals-popular-email-header-analysis-tools%2F "Share on X/Twitter") [ ](https://www.facebook.com/sharer/sharer.php?u=undefined%2Fblog%2F7-steps-verify-arc-seals-popular-email-header-analysis-tools%2F "Share on Facebook") [ ](https://reddit.com/submit?url=undefined%2Fblog%2F7-steps-verify-arc-seals-popular-email-header-analysis-tools%2F&title=7%20Steps%20To%20Verify%20ARC%20Seals%20Using%20Popular%20Email%20Header%20Analysis%20Tools%20 "Share on Reddit") [ ](mailto:?subject=7%20Steps%20To%20Verify%20ARC%20Seals%20Using%20Popular%20Email%20Header%20Analysis%20Tools%20&body=Check out this article: undefined%2Fblog%2F7-steps-verify-arc-seals-popular-email-header-analysis-tools%2F "Share via Email") ![ARC Seal Verification](https://media.mailhop.org/duocircle/spf-record-checker-0681-1784617393113.jpg) Organizations frequently forward emails through mailing lists, cloud email gateways, secure email services, and forwarding systems. During this process, legitimate messages may fail SPF or DKIM checks, causing **DMARC authentication to fail** even when the email is genuine. Authenticated Received Chain (ARC) was developed to preserve the original authentication results, allowing receiving [mail servers](https://www.cloudflare.com/learning/email-security/what-is-a-mail-server/) to make more informed trust decisions for forwarded emails. ARC preserves email authentication results as messages pass through multiple intermediaries, helping receiving mail servers evaluate forwarded emails more accurately. However, simply implementing ARC is not enough. Organizations should regularly verify ARC seals to ensure they are correctly signed, trusted, and functioning as intended. Fortunately, several email header analysis tools make it easy to verify ARC seals. This guide walks through seven essential steps to validate ARC seals using popular **email header analysis tools**, helping you strengthen email authentication, improve deliverability, and troubleshoot forwarding-related issues. ## What Are ARC Seals? _An ARC Seal (ARC-Seal) constitutes one of the three headers associated with ARC that are appended to an email by a secondary mail server._ These include: - ARC-Authentication-Results (AAR) - ARC-Message-Signature (AMS) - ARC-Seal (AS) Collectively, these headers maintain the **authentication status of an email** prior to its forwarding. The ARC-Seal applies cryptographic signatures to the preceding ARC data, guaranteeing that the authentication results remain intact as the message traverses various servers. Without ARC, [email forwarding](https://www.activecampaign.com/glossary/email-forwarding) frequently causes messages to fail DMARC validation because forwarding servers often **modify SPF alignment**. ARC preserves the original authentication results, allowing receiving mail servers to trust the authentication performed before the email forwarding process instead.![Spf Record 0682](https://media.mailhop.org/duocircle/spf-record-0682-1784617913109.jpg) ## Why Verify ARC Seals? Confirming ARC seals allows administrators to assess the trustworthiness of forwarded emails. Consistent verification of ARC seals provides organizations with several advantages: - Ensures the validity of ARC signatures. - Identifies any disruptions in authentication chains. - Aids in resolving email delivery challenges. - Enhances the likelihood of successful [inbox placement](https://www.zoho.com/zeptomail/glossary/what-is-inbox-placement.html). - Mitigates authentication **failures post-forwarding**. - Bolsters comprehensive email security measures. - Preserves the [domain reputation](https://kinsta.com/blog/domain-reputation/). For those overseeing platforms like Microsoft 365, [Google Workspace](https://en.wikipedia.org/wiki/Google%5FWorkspace), Proofpoint, Mimecast, or any other secure email gateway, integrating regular ARC verification into your email security processes is essential. ## Step 1: Collect the Full Email Header From the Original Message Start by collecting the complete email header from the original message, not from a screenshot, forwarded copy, or simplified view. ARC verification depends on raw message headers, including the **full sequence of Authentication-Results**, ARC-Authentication-Results, ARC-Message-Signature, and ARC-Seal header fields. In Outlook and Microsoft 365, administrators can retrieve headers from the message properties, message trace, or **security investigation tools**. If the message was filtered into the Junk Email folder or appears among quarantined messages, use Microsoft Defender for Office 365 to obtain the original header data. ### Why the Original Header Matters ARC is designed to account for legitimate email modification during routing. For example, an inbound email may pass through Proofpoint Protection Server, Proofpoint Essentials, Mimecast, Barracuda Email Gateway Defense, Barracuda Email Security Gateway, or Sophos Central Email Security before reaching Microsoft 365\. These systems may alter the message body or headers, which can break DKIM while preserving trust through **Authenticated Received Chain validation**. If you only inspect a copied or forwarded message, the ARC chain may be incomplete, making accurate header analysis impossible. ## Step 2: Identify the ARC-Authentication-Results, ARC-Message-Signature, and ARC-Seal Fields Next, locate the three ARC components in the message headers: - ARC-Authentication-Results - ARC-Message-Signature - ARC-Seal The ARC-Authentication-Results field records the authentication results observed by the intermediary. The ARC-Message-Signature signs selected message content and headers. The **ARC-Seal header seals** the ARC set and links it to the previous ARC instance.![Spf Record Check 0683](https://media.mailhop.org/duocircle/spf-record-check-0683-1784617934355.jpg) ### Key ARC Fields to Review Look for values such as: - i= — the ARC instance number - d= — the signing domain name or ARC sealer domain - s= — selector used for [DNS lookup](https://www.digicert.com/faq/dns/how-does-dns-lookup-work) - cv= — chain validation result - a= — algorithm used to sign the ARC set The ARC sealer domain is especially important. If the message passed through known **email security providers** such as pphosted.com, mimecast.com, barracudanetworks.com, or sophos.com, verify that the ARC seal was applied by the expected provider. In Microsoft 365, only configured trusted ARC sealers should influence downstream email authentication decisions. #### ARC-Seal Header Example A typical ARC-Seal header may include: ``` ARC-Seal: i=1; a=rsa-sha256; d=pphosted.com; s=selector; cv=none; b=... ``` The d= value identifies the ARC sealer domain, while cv= indicates whether the Authenticated Received Chain was validated at that point. ## Step 3: Check the ARC Chain Order and Instance Numbers ARC records must appear in a valid sequence. Each ARC set uses an instance number, such as i=1, i=2, and i=3\. The chain should be logical and continuous, with each ARC-Seal header protecting the **corresponding ARC-Message-Signature** and ARC-Authentication-Results. If an ARC set is missing, duplicated, or reordered, the ARC seal may fail validation. _A broken Authenticated Received Chain can indicate improper forwarding, malformed gateway behavior, or malicious tampering._ ### What a Healthy ARC Chain Looks Like A healthy ARC chain generally shows: - Instance numbers increasing by one - Matching ARC sets for each intermediary - A valid ARC-Message-Signature - A recognizable ARC sealer domain - Consistent authentication results across hops In Microsoft 365, ARC can also affect composite authentication. When ARC is trusted and validation succeeds, you may see results such as **compauth=pass with reason=130**, indicating Microsoft 365 accepted the trusted ARC context for the message. ## Step 4: Use Google Admin Toolbox Messageheader to Parse ARC Results Google Admin Toolbox Messageheader is a **convenient first-pass tool** for parsing message headers. Paste the full header into the tool to visualize routing hops, delays, Authentication-Results, SPF, DKIM, DMARC, and ARC-related fields. Although this tool is not specific to Microsoft 365, it helps quickly identify where the ARC seal was added, whether the ARC-Message-Signature appears complete, and which ARC sealer domain handled the message. ### What to Look for in Parsed Output Focus on these items: - Whether the ARC-Seal header is present - Whether each ARC set has matching instance numbers - Whether the sealer is one of your trusted ARC sealers - Whether the original [email authentication](https://www.duocircle.com/resources/email-authentication/) passed before forwarding - Whether **DMARC, SPF, or DKIM failed** only after legitimate modification #### Common Vendor ARC Sealing Examples You may see vendor ARC sealing from providers such as: - Proofpoint, Proofpoint Protection Server, Proofpoint Essentials, or pphosted.com - Mimecast or mimecast.com - Barracuda, Barracuda Email Gateway Defense, Barracuda Email Security Gateway, or barracudanetworks.com - Sophos, Sophos Central Email Security, or sophos.com These may be valid trusted ARC sealers if they are part of your **organization’s mail flow**.![Spf Record Tester 0684](https://media.mailhop.org/duocircle/spf-record-tester-0684-1784617970846.jpg) ## Step 5: Validate ARC Signatures With MXToolbox or Similar Header Analyzers After initial parsing, use MXToolbox or another Message Header Analyzer to perform deeper header analysis. These tools can help expose broken signatures, malformed headers, DNS selector issues, or suspicious routing behavior. ARC signature validation requires the receiving system to check [DNS records](https://www.ibm.com/think/topics/dns-records) for the **selector and domain name** used by the ARC sealer domain. If the [public key](https://www.coursera.org/in/articles/public-key) is missing, incorrect, or unavailable, the ARC-Message-Signature or ARC-Seal header may fail. ### Microsoft 365 and Defender Validation Context In Microsoft 365, Microsoft Defender for **Office 365 evaluates ARC** alongside broader email security signals, including anti-phishing policies, malware verdicts, impersonation checks, business email compromise indicators, and [phishing attacks](https://thehackernews.com/2026/06/china-linked-ta4922-expands-phishing.html) patterns. ARC does not replace SPF, DKIM, or DMARC; it preserves previous email authentication decisions when legitimate forwarding changes the message. For high-risk environments focused on breach prevention and ransomware protection, ARC should be treated as one signal within a layered control model, not as unconditional trust. ## Step 6: Confirm the ARC Seal Status, Domain Alignment, and Authentication Results Now compare the parsed ARC data with your **Microsoft 365 configuration**. _In the Microsoft Defender portal, go to Email & collaboration > Policies & rules > Threat policies > Email Authentication Settings to review ARC configuration where available_. This is where administrators can Add trusted ARC sealers or Remove trusted ARC sealers. You can also manage ARC configuration through Exchange Online PowerShell using PowerShell commands such as Set-ArcConfig. For example, [Exchange Online](https://www.techtarget.com/searchwindowsserver/definition/Exchange-Online) administrators may add approved vendor domains as trusted ARC sealers so Microsoft 365 can consider their ARC seal during authentication evaluation. ### Permissions Required in Microsoft 365 ARC configuration requires **appropriate admin permissions**. Depending on your tenant model, access may require Global Administrator, Security Administrator, or membership in Organization Management through [role based access control](https://www.upguard.com/blog/rbac). These roles are managed through Microsoft Entra, Exchange Online, and Microsoft Defender porta RBAC. To inspect or configure trusted ARC behavior, administrators often use: - Microsoft Defender portal - Exchange Online admin capabilities - Exchange Online PowerShell - Set-ArcConfig - Defender for **Office 365 policy pages** #### Trusted ARC Sealers and Domain Alignment The ARC sealer domain should align with a known gateway or provider in your mail flow. For example, if your organization routes inbound mail through Mimecast, then mimecast.com may be expected. If you use Barracuda, barracudanetworks.com may be expected. If you use Proofpoint, pphosted.com may appear. Do not add random senders as trusted ARC sealers. Only configure trusted ARC sealers for domains that **perform legitimate mail handling** for your organization. Misconfigured Add trusted ARC sealers settings can weaken [email security](https://www.duocircle.com/) by allowing questionable ARC assertions to influence delivery decisions.![Spf Validator 0685](https://media.mailhop.org/duocircle/spf-validator-0685-1784617991637.jpg) ## Step 7: Interpret Failures and Decide Whether the Message Can Be Trusted Finally, interpret the result in context. A failed ARC seal does not always prove the message is malicious, but it means the Authenticated Received Chain cannot be relied on. Similarly, a passing ARC seal does not guarantee the message is safe; it only confirms that the ARC chain and ARC-Message-Signature were validated according to the **sealer’s assertions**. ### How to Evaluate ARC Failure Scenarios Common failure causes include: - Missing or broken ARC-Seal header - Invalid ARC-Message-Signature - Unknown ARC sealer domain - Untrusted intermediary - **DNS selector lookup** failure - Message body changes after sealing - Misconfigured trusted ARC sealers - Failed DKIM, SPF, or DMARC before the ARC set was added Verify that the ARC sealer is configured as a trusted intermediary within your email security platform or mail server. Before trusting any ARC assertions, confirm that the provider is legitimate and aligns with your organization’s mail flow and security policies. A message is generally more trustworthy when the original SPF, DKIM, or DMARC authentication passes, the **ARC seal validates successfully**, and the ARC sealer belongs to a recognized email service or security provider. However, ARC should always be evaluated alongside other email authentication and security signals.![Spf Record Generator 0686](https://media.mailhop.org/duocircle/spf-record-generator-0686-1784618002596.jpg) _If the ARC chain fails, the sealer is unknown, or the message exhibits signs of spoofing, phishing, malware, or impersonation, treat it as suspicious_. Review the message headers, investigate the mail flow, and use your email security platform or [secure email gateway](https://www.duocircle.com/content/email-gateway/secure-email-gateway-definition/) to determine whether the message is legitimate. Regular ARC verification, combined with SPF, DKIM, and DMARC monitoring, helps organizations maintain trusted email authentication, improve deliverability, and identify forwarding-related issues before they **impact inbox placement**. For organizations using secure email gateways and email security solutions, routine ARC validation should be part of a comprehensive email authentication strategy. ![Brad Slavin](https://media.mailhop.org/dmarcreport/images/team/brad-slavin.jpg) Brad Slavin General Manager General Manager at DuoCircle. Product strategy and commercial lead across the email security portfolio. ## Secure your email infrastructure Protect, authenticate, and deliver. Contact our team to find the right solution. [Contact Sales](/contact/) [Explore Products](/products/) ## Related Articles [ tutorials How To Add A DKIM Record (V=DKIM1) In 5 Simple Steps Jun 24, 2026 ](/blog/how-to-add-dkim-record-vdkim1-in-5-simple-steps/)[ tutorials How to Configure SPF for Network Solutions (NetSol) Jun 18, 2026 ](/blog/how-to-configure-spf-for-network-solutions-netsol/)[ tutorials How To Perform An Email Blacklist Check Before Sending Large Campaigns Jul 2, 2026 ](/blog/how-to-perform-email-blacklist-check-before-sending-large-campaigns/) ```json {"@context":"https://schema.org","@type":"Organization","name":"DuoCircle LLC","url":"https://www.duocircle.com","logo":{"@type":"ImageObject","url":"https://www.duocircle.com/images/duocircle-logo.png"},"description":"DuoCircle is a portfolio of specialized email products covering protection, authentication, delivery, and routing. We deliver about 90% of category-leader capability at roughly half the price, backed by experts who own the outcome. 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