The Use of ARC in Email Security: All You Should Know

Authenticated Received Chain (ARC) is an email authentication protocol that preserves verification results when messages pass through intermediate servers like mailing lists or forwarding services.

Published as RFC 8617 in July 2019, the ARC protocol solves a critical problem: legitimate emails failing security checks simply because they were forwarded or sent through a group email service. Without ARC email authentication, your carefully crafted mailing list messages might land in spam — or never arrive at all.

If you run a mailing list for your club, association, or organization, you’ve likely experienced the frustration of members complaining they’re not receiving emails. The culprit is often DMARC forwarding failures — and ARC is the solution the email industry developed to fix it.

This guide explains how authenticated received chain technology works, why it matters for anyone managing group communications, and how it fits alongside existing email authentication protocols like DMARC, DKIM, and SPF.

What Is ARC Email Authentication?

ARC email authentication creates a verifiable chain of custody for messages as they travel across the internet. When an email passes through an intermediate server — such as a mailing list service — that server records the authentication status at that moment and cryptographically signs this information. The next server in the chain can then see not just the current authentication results, but the entire history of how the message was verified at each step.

Think of it like a package tracking system for email security. Each handler stamps the parcel with their verification, creating an unbroken record from sender to recipient. The ARC protocol adds three specific headers to accomplish this: ARC-Authentication-Results (capturing what passed or failed), the ARC message signature (signing the message content), and the email ARC seal (signing the entire chain to prevent tampering). Together, these components ensure that even if an email’s original authentication breaks during forwarding, the receiving server can see it was legitimate when it started its journey.

Major email providers, including Gmail, Microsoft Outlook, Yahoo, and Proton Mail, all support ARC validation, making it increasingly important for anyone sending group emails to ensure their mailing list provider implements this standard.

Why DMARC Forwarding Breaks Email Authentication

To understand the ARC benefits, you first need to grasp why email authentication fails in the first place. DMARC (Domain-based Message Authentication, Reporting and Conformance) relies on two underlying protocols: SPF checks whether an email came from an authorized server, while DKIM verifies that the message content hasn’t been altered since sending. When both of these pass and align with the sender’s domain, DMARC succeeds, and the email is trusted.

The problem emerges when emails are forwarded or sent through mailing lists. When your mailing list service receives a member’s message and redistributes it to the group, several things change. The sending server is now the mailing list’s infrastructure, not the original sender’s — so SPF fails because that IP address isn’t authorized for the original domain. Meanwhile, most mailing lists add useful modifications: a subject line prefix like “Club News”, an unsubscribe link in the footer, or reply-to adjustments. These alterations, however minor, break the DKIM signature because the message content no longer matches what was originally signed.

With both SPF and DKIM failing, DMARC fails too. If the original sender’s domain has a strict DMARC policy (p=reject), the email gets blocked entirely. Even with a quarantine policy, it likely ends up in spam. The result? Members of your sports club, professional association, or community group stop receiving legitimate communications — and you may never know unless someone complains. This is precisely where ARC email authentication becomes essential.

How ARC Works: The Three Email Authentication Headers

The authenticated received chain protocol adds three headers each time an email passes through an ARC-enabled server. Understanding these helps demystify how the system preserves trust across multiple hops.

ARC-Authentication-Results (AAR): This header captures a snapshot of the email’s authentication status at that moment. It records whether SPF, DKIM, and DMARC passed or failed when the intermediate server received the message. This is crucial because it preserves the original authentication verdict before any modifications occur.

ARC-Message-Signature (AMS): The ARC message signature functions similarly to a DKIM signature. It creates a cryptographic hash of the message headers and body as they exist at that point, binding the content to the ARC chain. If anyone tampers with the message after this signature is applied, validation will fail.

ARC-Seal (AS): The email ARC seal signs all the previous ARC headers, including any from earlier hops in the chain. This seal includes a critical “cv=” tag indicating chain validation status: “none” if this is the first hop, “pass” if all previous ARC signatures validated correctly, or “fail” if something in the chain couldn’t be verified.

When the email reaches its final destination, the receiving server validates the entire chain by checking each seal in sequence, confirming an unbroken record of authentication. This three-header system forms the foundation of how ARC email authentication preserves trust through complex email routing scenarios.

ARC vs DKIM: Understanding the Key Differences

Since both ARC and DKIM use cryptographic signatures, people often ask how they differ. The distinction is fundamental to understanding why both are necessary in modern email authentication protocols.

DKIM (DomainKeys Identified Mail) is applied by the original sender and validates that a message hasn’t been modified since it left their server. It’s a point-to-point authentication: “This message is exactly what the sender sent”. The weakness is that any change — even adding a mailing list footer — invalidates the signature. DKIM doesn’t account for legitimate intermediaries.

ARC, by contrast, is specifically designed for those intermediaries. Rather than replacing DKIM, the ARC protocol preserves DKIM’s results and adds its own layer of verification at each hop. Think of DKIM as the original seal on a letter, while ARC is the chain of custody documentation that explains what happened to that letter as it passed through various post offices.

The key technical difference appears in their headers. DKIM includes a “bh” tag containing a hash of the message body — any body modification breaks it. ARC’s signature structure uses instance numbers (“i=1”, “i=2”) to sequence multiple hops, and the “cv=” chain validation tag to indicate whether previous links in the chain are intact. ARC accepts that modifications will happen and focuses on documenting them rather than preventing them.

For organizations running mailing lists, the ARC vs DKIM distinction matters enormously. You can implement perfect DKIM signing, but without ARC support from your mailing list provider, your members’ messages may still fail authentication when redistributed to the group. This is also fundamentally different from DKIM re-signing, where the intermediate server replaces the original signature entirely — ARC preserves the original results while adding new verification layers.

How Mailing Lists Benefit from ARC Email

Mailing lists are perhaps the most common scenario where ARC email authentication proves essential. When a member sends a message to your club’s list address, it doesn’t go directly to other members — it passes through the list server, which modifies and redistributes it. This legitimate intermediate handling is exactly what causes DMARC forwarding problems.

With ARC enabled, the mailing list server performs authentication checks when it receives the incoming message, records those results in the ARC-Authentication-Results header, and then signs the outgoing message. When a member’s inbox provider receives the redistributed email and sees that DMARC fails (because the list server’s IP isn’t authorized for the original sender’s domain), it can check the ARC chain instead. If the chain validates and shows the message originally passed authentication, the inbox provider can choose to accept the email despite the DMARC failure.

This process delivers several concrete ARC benefits for organizations managing group communications. Members stop missing important emails simply because they happen to use inbox providers with strict spam filtering. Administrators spend less time troubleshooting delivery complaints. And the organization can encourage members to set strong DMARC policies on their own domains without worrying that this will prevent them from participating in group discussions.

Modern mailing list software has embraced this standard. Google Groups implements ARC, as does Mailman (the popular open-source list manager), Sympa, and commercial services. When evaluating any email list service, confirming ARC compliance should be on your checklist alongside features like moderation controls and member management.

Do Enterprises Need ARC If They Already Enforce DMARC?

The honest answer is that you probably don’t need to implement ARC yourself — but you absolutely need your email service providers to support it. Unless you’re running your own mail server infrastructure, your role is to choose providers who handle these email authentication protocols correctly on your behalf.

Signs that ARC support matters for your organization include: members reporting they’re not receiving mailing list messages; emails from your lists consistently landing in spam; members who work for large corporations or government bodies (which often have strict DMARC policies) being unable to participate in group discussions; or recurring complaints about email deliverability that you can’t trace to an obvious cause.

For most organizations — whether you’re a sports club, professional association, school parent group, or community organization — the practical step is to ask your mailing list provider about their ARC implementation. A quality provider will have this handled automatically. If your current service doesn’t support ARC and you’re experiencing DMARC forwarding issues, it may be time to consider alternatives.

When evaluating providers, look beyond just ARC to the full suite of email authentication. Do they sign outgoing messages with DKIM? Do they publish appropriate SPF records? Do they provide tools to check deliverability? The best services handle all of this without requiring technical expertise from administrators.

Steps to Implement and Validate ARC

While most organizations rely on their email service provider for ARC implementation, understanding the validation process helps when troubleshooting deliverability issues or evaluating potential providers. Here’s what a proper ARC implementation involves:

1. Server software support: The mail server must run software capable of generating and validating ARC headers. Popular platforms like Postfix (with OpenARC), Microsoft Exchange, and commercial solutions now include ARC support.
2. DNS key publication: Like DKIM, ARC requires cryptographic keys. The public key must be published in DNS so receiving servers can verify signatures.
3. Chain validation logic: When receiving forwarded messages, servers must validate the entire ARC chain — checking each seal in sequence and verifying the cv= tags indicate "pass" throughout.
4. Trust decisions: Receiving servers must decide which ARC sealers to trust. Major inbox providers maintain lists of trusted intermediaries.
5. Testing and monitoring: Use an email authentication checker to verify your mailing list messages include properly formed ARC headers. Regular testing ensures the implementation continues working correctly.

For administrators who want to verify their provider’s ARC implementation, send a test message through your mailing list and examine the received email headers. Look for ARC-Authentication-Results, ARC-Message-Signature, and ARC-Seal headers with sequential instance numbers (i=1, i=2, etc.) and cv=pass values.

Does ARC Improve Deliverability Measurably?

The impact of ARC on email deliverability is most pronounced in specific scenarios: forwarded messages and mailing list traffic, where traditional authentication would otherwise fail. For these use cases, ARC can make the difference between inbox delivery and rejection.

Since February 2024, Google has required bulk senders who forward email (including mailing list operators) to add ARC headers to outgoing messages. This requirement reflects the measurable impact ARC has on legitimate email delivery. According to Google’s sender guidelines, forwarded messages that pass ARC validation when they previously failed authentication can be accepted based on the preserved chain of trust.

The practical improvement depends on your audience composition. Organizations with members using enterprise email systems (which typically enforce strict DMARC policies) will see the most benefit. If your mailing list includes participants from banks, government agencies, healthcare organizations, or large corporations, ARC support can significantly reduce delivery failures to these recipients.

However, ARC is not a silver bullet. It complements rather than replaces proper email authentication fundamentals. Your domain still needs correctly configured SPF records, DKIM signing, and a DMARC policy. ARC simply ensures these protections remain meaningful when legitimate forwarding occurs. Learn more about improving your email sender reputation to maximize deliverability.

Common ARC Misconfigurations to Avoid

While end users don’t typically configure ARC themselves, understanding common problems helps when troubleshooting deliverability issues or evaluating providers.

Incomplete implementation: Some services add ARC headers without properly validating incoming chains, or they seal messages without including all required components. An email authentication checker can reveal whether outgoing messages from your mailing list include properly formed ARC headers.

Stripping existing headers: Another issue arises when intermediate servers strip existing ARC headers rather than adding to the chain. This breaks the chain of custody and defeats the purpose entirely. Quality mailing list providers preserve incoming ARC chains and add their own link, maintaining the full authentication history.

Trust configuration issues: Inbox providers must decide which ARC sealers to trust — accepting ARC results from any server would create a security vulnerability. Major providers like Gmail and Microsoft maintain lists of trusted sealers, which is another reason to use established mailing list services rather than obscure alternatives.

Neglecting fundamentals: Some organizations focus on ARC while neglecting the fundamentals. ARC complements SPF, DKIM, and DMARC — it doesn’t replace them. An email that never had proper authentication in the first place won’t be saved by ARC. Ensuring your sending domain has correct DNS records for all these email authentication protocols remains the essential first step.

Where Does ARC Fit with SPF, DKIM, and DMARC?

Understanding how ARC integrates with existing email authentication protocols helps clarify when and why it’s needed. Each protocol serves a distinct purpose in the email security stack:

• SPF (Sender Policy Framework): Authorizes which IP addresses can send email for your domain. Fails when emails are forwarded because the forwarding server’s IP isn’t in the original domain’s authorized list.
• DKIM (DomainKeys Identified Mail): Signs message content to verify it hasn’t changed. Fails when mailing lists add headers, footers, or modify subjects.
• DMARC (Domain-based Message Authentication, Reporting & Conformance): Coordinates SPF and DKIM, specifying what to do when authentication fails. Enforces policies but can reject legitimate forwarded mail.
• ARC (Authenticated Received Chain): Preserves the original authentication results before modifications occur, allowing receivers to trust messages that would otherwise fail DMARC.

Think of these protocols as layers. SPF and DKIM provide the foundation. DMARC adds policy enforcement. ARC extends this foundation to handle the reality that legitimate emails often pass through intermediaries. Together, they create a comprehensive approach to email authentication that works for both direct sending and complex routing scenarios like mailing lists. You can learn more about setting up DMARC vs DKIM in our detailed comparison guide.

Frequently Asked Questions

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