ARC Email Authentication Explained: How the Authenticated Received Chain Protects Forwarded Emails

Estimated reading time: 5 minutes

Email authentication keeps evolving, and as forwarding, routing, and mailing-list activity grow, traditional mechanisms like SPF and DKIM struggle to preserve authentication results. This is where ARC (Authenticated Received Chain) steps in. ARC helps receivers understand whether an email was originally legitimate, even if it passed through multiple intermediaries.

If you’ve ever wondered why forwarded emails often break SPF or why mailing list messages get flagged despite being legitimate, ARC is the protocol built to fix this.

What Is ARC (Authenticated Received Chain)?

ARC, or Authenticated Received Chain, is an email authentication protocol designed to preserve and communicate the authentication results of a message as it travels between different email systems.

When an email passes through intermediaries (like forwarding services, mailing lists, CRM systems, help desks, or automated gateways), traditional authentication standards (SPF, DKIM, DMARC) often break. This can cause legitimate emails to fail verification and land in spam.

ARC fixes this by allowing each server in the delivery chain to:

1. Record the message’s original authentication results
2. Sign those results using cryptographic seals
3. Pass the chain forward to the next receiving server

The final inbox provider (like Gmail, Yahoo, or Outlook) can then check the chain, verify its validity, and decide whether the email was originally authenticated before it was modified.

Why ARC Matters for Modern Email Deliverability

ARC is becoming crucial because the way emails travel has changed. Users forward emails from work accounts to personal inboxes, companies use ticketing systems that modify messages, and mailing lists rewrite headers. All of this breaks traditional authentication.

ARC solves that problem by:

• Preserving authentication results even when forwarding changes to an email
• Helping inbox providers identify legitimate messages that would otherwise fail DMARC
• Supporting mailing lists without forcing them to reconfigure authentication
• Reducing false positives where legitimate messages end up in spam
• Improving domain reputation stability

Put simply, ARC acts as a trust chain, showing the full path a message took and proving it wasn’t tampered with.

How ARC Works: Key Components Explained

ARC relies on multiple headers that each carry a piece of authentication logic. Together, they form a cryptographically signed chain.

1. ARC-Authentication-Results.

This header captures the original authentication results of SPF, DKIM, and DMARC before the message is modified by an intermediate hop.

2. ARC-Message-Signature (AMS).

Similar to DKIM, the AMS signs the message content and selected headers. It proves what the message looked like before modifications.

3. ARC-Seal (AS).

The ARC-Seal cryptographically signs the entire ARC set to ensure no one can alter the chain. It verifies:

• ARC-Authentication-Results
• ARC-Message-Signature
• Previous ARC-Seal instances

Each server that handles the message adds its own “ARC instance,” increasing the chain’s length.

ARC vs DKIM vs SPF vs DMARC

ARC does not replace current authentication standards. Instead, it supports them.

• SPF: Breaks when a message is forwarded
• DKIM: Breaks when message bodies are modified
• DMARC: Fails when SPF/DKIM breaks
• ARC’s purpose is to preserve authentication results rather than authenticate the message by itself.

It acts as a complementary protocol, especially useful when receiving systems evaluate authentication inconsistently because of forwarding or routing changes.

Who Needs ARC the Most

ARC is essential for any system that modifies or forwards messages:

• Email forwarding services
• Outlook/Gmail auto-forwarded mail
• Ticketing and support desk platforms
• CRMs and marketing automation systems
• University or corporate routing infrastructure
• Mailing lists and listservs

If your recipients frequently forward emails or use these systems, implementing ARC can improve deliverability and reduce DMARC-related rejections.

ARC and Deliverability: Why It Affects Inbox Placement

Mailbox providers increasingly rely on ARC to make smarter decisions about messages with broken SPF/DKIM. Without ARC, they often classify them as suspicious.

With ARC:

• The original authentication results remain visible
• Providers can trust the source even if intermediaries changed the message
• Fewer messages are mistakenly routed to spam
• DMARC alignment becomes more reliable in forwarding scenarios

In the long run, ARC contributes to a more stable sender reputation and higher inbox placement.

ARC works best when your DMARC policy is properly configured and monitored. Use GlockApps DMARC Analyzer to track authentication results and detect forwarding issues.

Conclusion

ARC (Authenticated Received Chain) is now a key piece of modern email authentication infrastructure. It preserves SPF, DKIM, and DMARC results through forwarding, routing, and third-party processing. As more organizations adopt stricter DMARC policies, ARC ensures legitimate emails maintain trust and reach the inbox, even when altered by intermediaries. To fully optimize ARC’s impact, continuous monitoring is critical.

Gain insights into ARC chains, DMARC alignment, and authentication performance with GlockApps’ DMARC tool, which is designed to help you improve inbox placement and protect your domain.

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