What is the primary function of the ARC-Authentication-Results header?

The ARC-Authentication-Results header captures and records the original authentication results (SPF, DKIM, DMARC) of an email before it undergoes any changes from forwarding services or mailing lists.

How does 'arc-status' help email deliverability?

When 'arc-status' shows 'pass', it indicates that a forwarded email's authentication chain is valid, allowing receiving servers to trust the original sender and deliver the message to the inbox, despite potential intermediate modifications.

Can 'arc-status' be 'none' even if ARC headers are present?

Yes, 'arc-status' can be 'none' if ARC-Seal headers are present but none of them are valid, indicating a broken or untrustworthy ARC chain.

Why is ARC important alongside DMARC?

ARC is crucial for DMARC because it provides historical authentication data that allows DMARC to validate emails even after they have been legitimately modified by forwarding. This prevents legitimate emails from failing DMARC checks and being rejected or marked as spam.

What is the 'arc-status' field in the ARC-Authentication-Results header? - ARC - Email authentication - Knowledge base

An email envelope with an Authenticated Received Chain seal

Email authentication protocols like SPF and DKIM are vital for verifying sender identity and preventing spoofing. However, these protocols can break when emails are forwarded or sent via mailing lists, leading to legitimate messages failing authentication checks. This is where Authenticated Received Chain (ARC) steps in, providing a way to preserve authentication results across multiple mail server hops.

The ARC-Authentication-Results header is a crucial component of ARC, containing the original authentication results from the moment an email first entered the ARC chain. Within this header, the 'arc-status' field provides a concise summary of the chain's validation status, telling receiving servers whether the chain is valid and trustworthy.

Understanding ARC and its headers

The challenge with email forwarding and mailing lists is that they often modify email headers or content in ways that invalidate SPF and DKIM signatures. This can cause perfectly legitimate emails to be flagged as suspicious by subsequent receiving mail servers, leading to delivery issues. ARC was developed to address this by allowing intermediate servers to attest to the email's original authentication state.

ARC introduces three main header fields: ARC-Authentication-Results, ARC-Message-Signature, and ARC-Seal. Each plays a specific role in creating a verifiable chain of custody for an email. The ARC-Authentication-Results header serves to capture the original authentication results before any modifications might occur.

It contains a snapshot of the email's authentication status (SPF, DKIM, DMARC) at each point where a new ARC-Seal is added. This allows subsequent recipients to see the historical authentication journey of an email, even if intermediate servers altered it. This context is crucial for making informed delivery decisions, especially when DMARC policies are in effect.

How ARC benefits email flow

Preserves legitimacy: Ensures forwarded emails or messages from mailing lists don't fail authentication checks due to transit modifications.
Maintains DMARC compliance: Helps receiving servers make correct DMARC decisions by providing a trusted history.
Increases deliverability: Reduces the chances of legitimate emails landing in spam folders.

The 'arc-status' field explained

The 'arc-status' field is found within the ARC-Authentication-Results header and is critical for understanding the validity of the ARC chain. This field, often abbreviated as 'cv' (chain validation), summarizes the outcome of the ARC chain validation process performed by the last ARC-enabled hop. Essentially, it tells you if the signed ARC chain is intact and trustworthy.

The possible values for 'arc-status' are defined in RFC 8617, section 3.2.1, and include:

None: No ARC-Seal headers were present, or none were valid.
Pass: The ARC chain validated successfully, meaning all ARC-Seal headers in the chain were correctly signed and verified.
Fail: The ARC chain failed validation at some point, indicating a break or tampering within the chain.

Example ARC-Authentication-Results headertext

ARC-Authentication-Results: i=1; mx.google.com; arc-status=pass dmarc=pass (p=none sp=none dis=none) smtp.from=example.com

Email servers validating ARC chain with a 'pass' status

Impact on email deliverability and trust

A 'pass' status for 'arc-status' is a strong signal to receiving mail servers that an email, despite having SPF or DKIM failures due to forwarding, originated from a legitimate source. This allows the receiving server to re-evaluate the DMARC policy with the preserved authentication results, significantly improving the chances of delivery to the inbox rather than being sent to spam or blocked entirely. Without a valid ARC chain, such emails would likely fail authentication and face negative consequences.

For organizations using DMARC, 'arc-status' is indispensable. It bridges the gap for legitimate emails that would otherwise fall victim to strict DMARC policies in forwarding scenarios. By providing a verified history of authentication, ARC prevents false negatives, ensuring that important communications reach their intended recipients. Monitoring your DMARC reports will show you the impact of ARC on your email flow.

Scenario	Without ARC	With ARC ('arc-status=pass')
Forwarded email	SPF/DKIM often break, leading to DMARC failure.	Original authentication preserved, aiding DMARC validation.
Deliverability	High risk of emails being sent to spam or rejected.	Increased likelihood of inbox delivery for legitimate mail.
Sender reputation	Can be negatively impacted by DMARC failures.	Protected by providing context for authentication issues.

Proactive monitoring for optimal email health

Understanding and configuring ARC is only half the battle. To truly leverage its benefits and ensure robust email deliverability, proactive monitoring of ARC validation results is essential. Regularly checking the 'arc-status' in your email headers and DMARC reports helps identify any issues in the authentication chain and allows for timely troubleshooting. This helps maintain a healthy email ecosystem, protecting your domain's reputation.

Platforms like Suped offer comprehensive DMARC monitoring that includes insights into ARC. Our AI-powered recommendations tell you exactly what actions to take to fix authentication issues, while real-time alerts ensure you're immediately notified of potential problems. With a unified platform for DMARC, SPF, and DKIM, you get a complete picture of your email authentication, helping you maintain optimal deliverability and protect against spoofing and phishing attacks.

Unmonitored ARC

Blind spots: Unaware of ARC chain breaks or validation failures.
Delayed detection: Discover deliverability issues only after complaints or significant impact.
Reactive fixes: Troubleshooting becomes reactive, leading to longer downtimes.

Monitored ARC with Suped

Full visibility: Clear insights into ARC validation status across all email flows.
Early warnings: Real-time alerts notify you of any ARC failures or anomalies.
Proactive solutions: AI-driven recommendations guide immediate, effective fixes.

Strengthen your email ecosystem

The 'arc-status' field in the ARC-Authentication-Results header is a small but powerful indicator of an email's authentication journey. It provides crucial context to receiving mail servers, helping them distinguish legitimate forwarded emails from malicious spoofing attempts. Understanding and utilizing ARC is a key step in building a resilient and trustworthy email infrastructure.

For complete email security and deliverability, integrate ARC into a broader strategy that includes robust DMARC, SPF, and DKIM implementation. With Suped, you gain powerful tools like SPF flattening and real-time alerts to manage these complexities, ensuring your emails consistently reach the inbox.