What is the primary problem ARC solves for mailing lists and forwarders?

ARC addresses the problem of legitimate emails from mailing lists and forwarding services failing DMARC authentication. Modifications made by these intermediaries typically break SPF and DKIM, causing DMARC to treat the email as suspicious, even if it's not spoofed.

How does ARC prevent DMARC failures caused by forwarding?

ARC creates a verifiable chain of custody by having each intermediary (forwarder or mailing list) sign the email's authentication state with an ARC-Seal. This allows the final recipient's mail server to see the original authentication results and trust that any subsequent DMARC failures are due to legitimate modifications, not malicious activity.

Does ARC replace SPF, DKIM, or DMARC?

No, ARC does not replace SPF, DKIM, or DMARC. It is an extension that works alongside these protocols. ARC fills a specific gap in the DMARC ecosystem by providing context for messages that undergo legitimate changes in transit, enabling DMARC to make more accurate judgments.

What happens if an intermediary in the email path is not ARC-enabled?

If an intermediary is not ARC-enabled, the ARC chain will be broken at that point. This means that subsequent receivers cannot fully verify the email's authentication history through ARC, and the email may still be subject to DMARC failures if SPF or DKIM are broken by that non-ARC-enabled hop.

How can I monitor ARC results for my domain?

You can monitor ARC results through a DMARC reporting and monitoring tool like Suped. These platforms collect and analyze DMARC reports, including details on ARC validation, providing insights into your email authentication status and deliverability.

Does ARC address issues with mailing lists and forwarders? - ARC - Email authentication - Knowledge base

An illustration showing an email flow, initially breaking DMARC with forwarding, then demonstrating ARC creating a secure, unbroken chain of authentication.

Email authentication protocols like SPF, DKIM, and DMARC are crucial for verifying sender identity and preventing spoofing. However, they face a significant challenge when emails are modified in transit, particularly through mailing lists or forwarding services. These legitimate alterations can cause DMARC authentication to fail, leading to emails being rejected or sent to spam folders, even if they are from a trusted source.

The core issue is that DMARC expects a direct path and unaltered headers from sender to recipient. When an email passes through an intermediary, like a forwarding service or a mailing list, the email's headers can be rewritten, or the message content subtly modified. This breaks the cryptographic signatures that DKIM relies on, or changes the sending domain in a way that SPF can't validate against the original sender's record.

This is where Authenticated Received Chain (ARC) steps in. ARC is designed specifically to address these DMARC challenges by providing a way to preserve email authentication results across multiple hops or intermediaries. It establishes a verifiable chain of custody, ensuring that legitimate emails, even after modification by forwarders or mailing lists, can still pass DMARC checks at the final destination.

Does ARC address issues with mailing lists and forwarders?

The challenge of email forwarding

Traditional email authentication protocols, SPF and DKIM, are robust but rigid. SPF (Sender Policy Framework) verifies the sending IP address, while DKIM (DomainKeys Identified Mail) uses cryptographic signatures to ensure the email hasn't been tampered with. The problem arises because email forwarding often involves a new server sending the email, which can break SPF by changing the sending IP. Additionally, mailing lists frequently modify email headers, footers, or even the body, which invalidates the original DKIM signature.

When SPF and DKIM fail, DMARC (Domain-based Message Authentication, Reporting, and Conformance), which relies on these two, will also fail. This leads to legitimate emails being flagged as suspicious, especially when the DMARC policy is set to quarantine or reject. This can significantly impact deliverability and cause frustration for users who rely on email forwarding or subscribe to mailing lists. Understanding how these issues arise is critical for anyone managing email delivery, as detailed in our guide on email forwarding and DMARC policies.

For legitimate senders, these DMARC failures can result in their important messages not reaching the inbox, being sent to the spam folder, or even being blocked entirely by recipient mail servers. This is particularly problematic for services like Google Groups or other legitimate forwarders, which are common communication channels. Fortunately, ARC was developed to bridge this gap and provide a solution to ensure these emails are still trusted.

How ARC preserves authentication

ARC works by providing an authenticated chain of custody for an email. When an email passes through an ARC-enabled intermediary, that intermediary (like a mailing list server or a forwarding service) will add a new set of ARC headers to the email. These headers attest to the email's authentication status at the point it was received by the intermediary. This includes an ARC-Seal header, which is a digital signature over the previous authentication results and the entire message (including previous ARC headers). This process allows subsequent receivers to verify the entire chain.

The key components of ARC are the ARC-Authentication-Results (AAR) header, which summarizes the authentication results at that hop, and the ARC-Message-Signature (AMS) and ARC-Seal headers. The ARC-Seal is critical because it binds the previous authentication state to the current message, forming a verifiable chain. When a receiving mail server processes an email with ARC headers, it can reconstruct the authentication history and determine if any DMARC failures are due to legitimate forwarding or malicious activity. This mechanism does not re-authenticate the email but rather vouches for its state at each hop. To learn more about implementation, see our article on how to implement ARC.

Example of an ARC-Seal headertext

ARC-Seal: i=1; a=rsa-sha256; t=1678886400; cv=none;
 s=arcselector; d=example.org;
 bh=AbCDEF123...;
 h=From:To:Subject:Date:Message-ID:Content-Type;
 b=GHIJKL456...

Essentially, ARC acts as an attestation of previous authentication results. It allows a mail receiver to trust that if a message failed DMARC after being handled by a legitimate intermediary, it was not because the original sender was spoofed, but because the legitimate intermediary modified the message in a standard way. This distinction is crucial for improving email deliverability for forwarded messages while maintaining DMARC's security benefits.

ARC and mailing lists specifically

Mailing lists are a prime example of where ARC provides immense value. Many mailing lists, like those commonly found in academic or open-source communities, modify emails by adding footers, prepending subject lines, or even changing the 'From' address to the list's address. These modifications inherently break DKIM signatures and SPF alignment. Without ARC, these legitimate emails would frequently fail DMARC and be treated as spam, disrupting communication. According to Fastmail, ARC is an experimental standard for authenticating mail in these complex scenarios.

With ARC in place, a mailing list server can sign the email with its own ARC-Seal, indicating that it received the email legitimately and then made the necessary modifications. The final recipient's mail server can then check this ARC chain. If the ARC chain is valid, the DMARC policy can be overridden or informed by the trusted ARC results, preventing legitimate emails from being incorrectly flagged as spam. This allows organizations to enforce strict DMARC policies (like p=reject) without inadvertently penalizing subscribers who use forwarding services or participate in mailing lists. This is also important for understanding how ARC prevents email spoofing.

Without ARC

Authentication breaks: Forwarded emails often fail SPF/DKIM due to changes in IP or headers.
DMARC failure: Legitimate messages incorrectly categorized as spam or rejected.
Lost trust: Reduced sender reputation and deliverability.

With ARC

Preserved authentication: ARC-enabled intermediaries provide an audited chain.
DMARC success: Legitimate messages pass DMARC, even after modification.
Enhanced trust: Improved sender reputation and deliverability.

While ARC significantly improves deliverability for mailing lists and forwarders, it's not a silver bullet. Its effectiveness relies on all intermediaries in the chain being ARC-enabled, which isn't always the case. However, with major email providers increasingly adopting ARC, its impact continues to grow, helping to secure the email ecosystem while maintaining the functionality of essential services like mailing lists.

The benefits and broader implications of ARC

ARC's primary benefit is ensuring that DMARC does not inadvertently penalize legitimate email flows. It preserves the integrity of authentication information across multiple hops, making it easier for recipient mail servers to make informed decisions. This allows senders to deploy stricter DMARC policies, such as p=reject, with greater confidence, knowing that messages from reputable sources won't be blocked simply because they were forwarded or sent via a mailing list. This also addresses concerns about how ARC validates the 'From' address.

DMARC monitoring with ARC insights

Implementing and monitoring ARC requires a comprehensive DMARC reporting tool. Suped is specifically designed to provide clear, actionable insights into your email authentication, including ARC results. Our AI-powered recommendations tell you exactly what to do to fix issues and strengthen your policy. You'll receive real-time alerts and benefit from a unified platform for DMARC, SPF, and DKIM monitoring, alongside blocklist and deliverability insights. Suped also offers advanced features like SPF flattening and a dedicated MSP and multi-tenancy dashboard, making it the ideal solution for both individual businesses and agencies managing multiple domains.

It is important to remember that ARC does not replace SPF, DKIM, or DMARC. Instead, it works as an extension, addressing a specific gap in DMARC's effectiveness when emails undergo legitimate modifications in transit. This cooperative approach helps maintain the overall security of the email ecosystem while improving the reliability of email delivery for common use cases like mailing lists and forwarding. To gain more insight, refer to our page Does ARC replace DMARC or SPF/DKIM?

An illustration showing a perfectly authenticated email flow, with green checkmarks for SPF, DKIM, and DMARC, all enabled by a strong ARC chain.

Conclusion: Enhancing email trust with ARC

ARC has emerged as a vital protocol for ensuring DMARC's continued effectiveness in a complex email environment. By providing a secure audit trail for authentication results across intermediaries, it successfully addresses the inherent issues that mailing lists and forwarding services pose to traditional email authentication. This allows organizations to maintain strong security postures with DMARC without sacrificing the utility of these common email practices, ultimately leading to better email deliverability and a more trustworthy communication landscape.

As email continues to evolve, protocols like ARC highlight the ongoing effort to balance robust security with practical functionality. For senders, this means embracing ARC alongside DMARC, SPF, and DKIM to maximize deliverability and protect their brand reputation. The adoption of ARC by major mail providers is a testament to its importance in solving these forwarding issues, as highlighted by Halon's article on DMARC forwarding issues with ARC.

For domains looking to implement DMARC or improve their existing policies, monitoring tools that provide visibility into ARC results are invaluable. Suped offers advanced DMARC monitoring that helps you navigate these complexities, ensuring your emails reach their intended recipients without being caught in the authentication traps of forwarding or mailing lists. Our platform simplifies the entire process, from initial setup to ongoing optimization, making DMARC accessible and effective for everyone.