The Authenticated Received Chain (ARC) is a crucial email authentication protocol, especially for mail that is forwarded through intermediate servers. It helps preserve the original email authentication results, such as SPF and DKIM, as a message travels through various mail handlers before reaching its final destination. Without ARC, legitimate emails that are forwarded might fail DMARC checks, leading to them being marked as spam or rejected.
Within the ARC system, the 'cv=pass' result is a significant indicator. It means that the receiving mail server successfully validated the ARC chain associated with an incoming email. This validation confirms that the email's authentication results, as recorded by previous ARC-signing intermediaries, have not been tampered with and are trustworthy.
Understanding this 'cv=pass' result is essential for anyone managing email deliverability, as it directly influences how DMARC policies are applied to forwarded messages. It acts as a bridge, allowing the recipient's mail server to make an informed decision about the email's authenticity, even when direct SPF or DKIM checks might otherwise fail due to legitimate mail flow.
The role of ARC in email authentication
How ARC preserves authentication
ARC works by creating a 'chain' of authentication results. Each intermediary server that modifies an email, such as a mailing list or a forwarding service, adds an ARC-Seal header and an ARC-Authentication-Results header. The ARC-Seal header contains a cryptographic signature of the message and the previous ARC headers, while the ARC-Authentication-Results header includes the authentication results (SPF, DKIM, DMARC) from that specific hop. This means subsequent receivers can inspect this chain to verify the email's authenticity at its origin. You can read more about ARC in general on Postmark's blog.
When an email passes through several servers, each adding its own ARC-Seal, a chain forms. The final receiving mail server then evaluates this ARC chain to determine if the message truly originated from the claimed sender. This is especially important because forwarding often modifies the email's headers or body, which can break SPF or DKIM signatures.
The 'cv' tag, standing for 'Chain Validation', is found within the ARC-Seal header. It indicates the status of the ARC chain validation process at the point an ARC-sealer added its seal. A 'cv=pass' result means the ARC-Seal and the entire chain of previous ARC-Seals were cryptographically verified as valid and untampered.
Understanding 'cv=pass' vs. 'cv=fail'
Understanding 'cv=pass' vs. 'cv=fail'
When an ARC-enabled mail server receives a message, it inspects the ARC-Seal header's 'cv' tag. A 'cv=pass' indicates that the signatures of all previous ARC-Seals in the chain were successfully validated against their corresponding ARC-Authentication-Results headers. This provides a strong signal of integrity to the final recipient, suggesting that the email has not been maliciously altered since the first ARC-signing intermediary.
Conversely, a 'cv=fail' (or 'cv=none', 'cv=invalid') means the ARC chain validation failed. This could be due to various reasons, such as a broken signature, missing ARC headers, or a server not being configured to sign ARC properly. For a deeper understanding of what happens when validation fails, you can explore what the 'cv=fail' result signifies.
The final receiving server uses the 'cv' result to decide whether to trust the authentication history provided by ARC. If it's 'cv=pass', the server can confidently use the original SPF and DKIM results from the first ARC-signing hop to inform its DMARC policy decision, even if the current SPF or DKIM checks are showing failures.
cv=pass: Indicates that the ARC chain, up to this point, has been successfully validated. The previous authentication results can be trusted.
i=1: This is the instance count of the ARC-Seal, indicating its position in the chain, with 1 being the most recent.
Impact on DMARC policy enforcement
Impact on DMARC policy enforcement
The primary purpose of ARC is to prevent legitimate emails from failing DMARC due to benign modifications made during forwarding. DMARC relies on SPF and DKIM alignment, which can easily break when a message is relayed through a mailing list, for example. Without ARC, these forwarded emails might be quarantined or rejected by the recipient's mail server, even if they originated from a valid sender.
When a receiving server sees an ARC chain with a 'cv=pass' result, it effectively says, 'I trust the previous server's assessment of this email's authenticity.' This allows the server to look at the original DMARC results, preserved in the ARC-Authentication-Results header, and treat the email as DMARC-compliant, even if current SPF or DKIM checks might not align. Microsoft Defender, for instance, explicitly leverages trusted ARC sealers to override DMARC failures for legitimate forwarded emails.
This mechanism is vital for maintaining email deliverability, especially for organizations that use mailing lists or forward emails frequently. Implementing a robust email authentication strategy, including DMARC with ARC support, is critical for ensuring your emails reach the inbox. You can learn more about the fundamentals of DMARC, SPF, and DKIM to further strengthen your email security.
Without ARC
Forwarding breaks SPF: IP address changes during forwarding invalidate SPF.
Mailing lists modify DKIM: Message body alterations break DKIM signatures.
DMARC fails: Legitimate forwarded emails fail DMARC, often leading to spam folders or rejection.
With ARC cv=pass
Authentication preserved: Original SPF/DKIM results are recorded and signed in the ARC chain.
Chain integrity verified: The 'cv=pass' indicates the chain is intact and trustworthy.
DMARC passes: Receivers can use the original results, allowing legitimate forwarded mail to pass DMARC.
Operational benefits for senders
Operational benefits for senders
For email senders, understanding and ensuring your emails are compatible with ARC, especially achieving a 'cv=pass' result when forwarded, has tangible benefits. It minimizes the risk of legitimate emails being erroneously flagged as suspicious or spam when they pass through intermediaries like discussion forums, newsletters, or personal forwarding rules. This directly contributes to better deliverability and a stronger sender reputation.
While you cannot directly control how intermediary servers implement ARC, ensuring your initial email authentication (SPF, DKIM, DMARC) is perfectly configured sets the stage for a healthy ARC chain. A well-configured DMARC record, for instance, will enable participating ARC signers to accurately record your initial authentication status, which is then preserved throughout the chain. Monitoring your ARC results, including the 'cv' tag, is a key part of maintaining this health. Specifically, checking the ARC-Authentication-Results header's arc-status field can provide valuable insights.
Ensuring secure email delivery
Ensuring secure email delivery
The 'cv=pass' result in ARC serves as a vital component in the ongoing effort to ensure secure and reliable email delivery. It offers a standardized way for email receivers to trust the authentication history of an email, even when the path it takes to the inbox is circuitous. This trust is fundamental in the fight against phishing and spoofing, as it helps distinguish legitimate forwarded mail from malicious attempts to impersonate senders.
To effectively leverage ARC and other authentication protocols, continuous monitoring of your DMARC reports is essential. Platforms like Suped's DMARC monitoring solution provide detailed insights into your email authentication results, including ARC data. Our AI-powered recommendations help you take actionable steps to fix issues and strengthen your policy. You'll receive real-time alerts for critical authentication failures and benefit from a unified platform that combines DMARC, SPF, and DKIM monitoring with blocklist and deliverability insights. Suped also offers SPF Flattening and an MSP and Multi-Tenancy Dashboard, making DMARC accessible and manageable for all, including through our generous free plan.
