How To Use ARC To Improve Forwarded Newsletter Authentication

Authenticated Received Chain is a way to preserve valid email authentication when a newsletter gets forwarded, filtered, or sent through a mailing list. Forwarding often breaks SPF alignment and can invalidate DKIM when intermediaries add banners, footers, or subject tags, which then causes DMARC to fail even for legitimate mail. With ARC enabled, the intermediary records the original Authentication-Results and adds signed ARC-Authentication-Results, ARC-Message-Signature, and ARC-Seal headers so the final mailbox provider can verify the chain and make a smarter delivery decision. The easy-to-miss catch is that ARC is typically added by the forwarder, so picking the right forwarding path matters as much as your own SPF, DKIM, and DMARC setup.

What is ARC (Authenticated Received Chain) in email authentication?

ARC vs SPF, DKIM, and DMARC roles

ARC (Authenticated Received Chain) is an email authentication framework that helps receivers understand what happened to a message as it passed through intermediaries like forwarders, mailing lists, and gateways. It does not replace SPF, DKIM, or DMARC. It complements them.

Here’s the practical difference:

• SPF checks whether the sending IP is allowed to send for a domain, based on the RFC5321.MailFrom (envelope-from). Forwarding often breaks this because the forwarder sends from its own IP.
• DKIM checks whether message content matches a cryptographic signature from a domain. Forwarding can break DKIM if an intermediary modifies the message (subject tags, footers, link rewrites, security banners).
• DMARC ties SPF and DKIM to the visible From domain and requires alignment. If forwarding breaks SPF and DKIM, DMARC can fail even for a legitimate newsletter.

ARC’s job is to preserve the authentication story. A forwarder that supports ARC can record the results it saw (SPF, DKIM, DMARC) and then sign and “seal” that record. The final mailbox provider can validate the ARC chain and decide whether to treat the message as trustworthy, even if DMARC fails at the last hop.

When ARC adds value for newsletters

ARC matters most when your newsletter is legitimately authenticated at the source, but gets altered or re-sent on the way to the subscriber’s inbox. Common examples include:

• Subscriber forwarding from a work inbox to a personal inbox.
• University or enterprise mail gateways that add security disclaimers.
• Mailing lists that re-send the message to a group.

In these cases, ARC can help the receiver see that your newsletter passed authentication earlier in the journey, before it was modified or forwarded. That can improve inbox placement and reduce false positives for phishing controls, especially for newsletters that subscribers routinely route through multiple systems. ARC is not a magic pass, though. Receivers still weigh sealer reputation, DMARC policy, and the overall spam signal profile before trusting ARC.

Why forwarded newsletters fail SPF, DKIM, or DMARC

Common forwarding paths that break alignment

Forwarding changes who is “sending” the message on the internet, even if the visible From address still shows your newsletter brand. That mismatch is the root of most authentication failures.

The most common paths that cause problems are:

• Mailbox auto-forwarding (for example, a user forwards their work mail to Gmail). The forwarder re-sends from its own servers, so the IP that delivers to the final mailbox is no longer one you authorized in SPF.
• Mailing lists and group re-mailers (Google Groups, listservs, ticketing systems). Many add list headers, modify the subject, or wrap the message, which can break DKIM.
• Security gateways (enterprise filters, university gateways). These often insert warning banners, rewrite URLs for safe browsing, or add attachments. Even a small change to a signed header or body can invalidate DKIM.
• “Forward as attachment” vs “forward inline”. Forwarding as an attachment can preserve the original message and signatures, while inline forwarding often rebuilds parts of the message and breaks them. Which method a client uses varies.

ARC exists because these forwarding paths are normal, especially for newsletters that get shared inside companies and communities.

Typical DMARC failure patterns after forwarding

After forwarding, DMARC typically fails in one of a few predictable ways:

1. SPF fails or loses alignment
   SPF might still pass for the forwarder’s domain, but DMARC cares about alignment with the visible From domain. If the forwarder uses its own envelope-from, SPF alignment with your From domain is lost.

2. DKIM fails due to message modification
   Your newsletter may have left your sending platform with a valid DKIM signature, but a forwarder that adds a footer, changes line wrapping, or rewrites links can cause DKIM to fail at the final recipient.

3. Only one of SPF/DKIM passes, but not aligned
   DMARC needs either SPF-aligned pass or DKIM-aligned pass. Forwarding can create a scenario where “something passes,” but not in a way DMARC accepts.

When you see “DMARC fail after forwarding,” it does not automatically mean your sending setup is wrong. It often means the message arrived through a path that changed the technical sender or the signed content. ARC can give the receiver verified evidence of what passed before that change happened.

ARC headers explained: AAR, AMS, and AS

What Authentication Results (AAR) records

ARC-Authentication-Results (AAR) is the “receipt” an intermediary writes down about what it observed when it received the message. It captures authentication outcomes like SPF, DKIM, and DMARC as they looked at that hop, plus details like which domain passed and which identity was evaluated.

AAR also includes an instance tag like i=1, i=2, and so on. Each time an ARC-aware system handles and re-sends the message, it adds a new ARC set with the next instance number. This is what turns ARC into a chain rather than a single stamp.

In practice, AAR is the piece that lets a mailbox provider say, “This newsletter did authenticate earlier, before it went through forwarding or rewriting.”

What Message Signature (AMS) signs

ARC-Message-Signature (AMS) is a DKIM-like signature that cryptographically signs the message “as it exists right now” at that hop. It covers the message body and selected headers (listed in the h= tag), which is how the intermediary can vouch for the content it is passing along.

A key detail: AMS is designed so later systems can still add their own ARC headers without breaking earlier AMS signatures, because ARC-related headers are not included in the AMS signed header list. This helps ARC survive multi-hop forwarding.

What Seal (AS) proves at each hop

ARC-Seal (AS) is the chain binder. It signs the ARC set and links it to prior ARC sets, so a receiver can detect missing hops or tampering. AS also carries the chain validation result in cv= (for example, pass or fail), which tells downstream validators whether the chain looked intact at the time of sealing.

If you want the exact field definitions and signing rules, the canonical reference is RFC 8617.

ARC validation at the final mailbox provider

Chain verification and cv results

When a forwarded newsletter arrives at the final mailbox provider, the receiver can validate the ARC chain before deciding what to do with the message. Validation is mainly about integrity: does the message contain a complete set of ARC headers at each hop, are the instance numbers continuous, and do the signatures and seals verify?

If the chain checks out, ARC gives the receiver a trustworthy record of what SPF, DKIM, and DMARC looked like earlier in the path, before forwarding or rewriting occurred. If the chain does not check out, ARC is treated as if it is not present.

Common cv values and what they mean

The cv= tag appears in each ARC-Seal. It summarizes how the sealer evaluated the chain that existed before it added its own seal:

• cv=none: No prior ARC chain existed (typical on the first ARC hop, i=1).
• cv=pass: The prior ARC chain validated successfully, so this hop is extending an intact chain.
• cv=fail: The prior ARC chain did not validate. Per the spec, a failing chain should not be relied on, and subsequent sealers are expected to stop sealing once they see a most-recent cv=fail.

How receivers decide whether to trust ARC

Even with a passing ARC chain, receivers still make a judgment call. ARC is an input, not an override. In practice, mailbox providers weigh:

• Who sealed it (the sealer’s domain and reputation).
• What the AAR says (did DKIM or SPF align and pass at an earlier hop?).
• Whether the current message still looks safe (content, URLs, engagement signals, and abuse patterns).
• Your DMARC policy and how severe the failure is at the final hop.

This is why ARC works best when your newsletter is already well-authenticated at the source, and the forwarder is a credible, consistently behaving ARC sealer.

ARC deployment options for newsletter senders and forwarders

Where ARC sealing should happen

ARC sealing should happen at the intermediary that changes the delivery path or the message. In newsletter land, that is usually the forwarder: a corporate gateway, a mailing list, a help desk platform, or a user’s mailbox provider that re-sends mail onward.

If you are the newsletter sender, ARC is rarely something you “turn on” at the sending platform and call it done. Your primary job is still to publish and maintain solid SPF, DKIM, and DMARC for the domain in your From address. ARC becomes valuable when a downstream system takes responsibility for preserving those good results through forwarding.

If you do operate an intermediary (for example, you host inbound mailboxes and forward mail to other providers), you are a strong candidate to become an ARC sealer.

Configuring MTAs and gateways to add ARC

Most ARC deployments look like “verify then seal”:

1. Your edge MTA receives the message.
2. It runs SPF/DKIM/DMARC checks and records the results.
3. If policy allows, it adds an ARC set (AAR, AMS, AS) and then relays the message.

Implementation options depend on your stack, but the pattern is consistent: ARC sealing is typically added via a milter/content filter or a gateway feature that can both validate and sign. The must-have pieces are key management (private key storage), DNS publishing for the ARC signing domain, and clear scoping of which traffic you seal.

When to avoid sealing (rewrites and munging)

Avoid ARC sealing when your system will unpredictably “munge” mail in ways that make your own ARC assertions unreliable. Common examples:

• Multiple downstream rewrites you do not control (subject tagging plus link rewriting plus footer insertion).
• Inconsistent canonicalization or transformations that vary by policy.
• Forwarding rules that re-envelope mail in a way that obscures what was actually evaluated.

A good rule: only seal when you can confidently verify first, and when your handling is stable enough that receivers can trust your ARC chain.

Trusted ARC sealers and abuse risks to watch

Evaluating sealer reputation and policy

ARC is only as useful as the intermediary that seals it. Receivers can validate the cryptography, but they still have to decide whether the sealer is careful and consistent enough to trust.

When you are evaluating a potential ARC sealer (or deciding whether your own infrastructure should seal), focus on a few practical questions:

• Do they verify before they seal? A trustworthy sealer authenticates inbound mail (SPF, DKIM, DMARC) and records real results in AAR, instead of sealing blindly.
• Is the sealing identity stable and accountable? The ARC signing domain should be a domain the operator controls and can protect. Frequent changes or unclear ownership are a red flag.
• Do they have anti-abuse controls? Rate limiting, malware filtering, and clear handling of obvious spoofing attempts matter. ARC can be abused if a sealer “launders” mail by vouching for messages it did not properly evaluate.
• Do they avoid sealing after a broken chain? If a message already has an invalid ARC chain, continuing to add seals can create confusion rather than clarity.

For newsletter senders, the key takeaway is simple: ARC helps most when it is added by intermediaries with strong hygiene. If a forwarder is sloppy, ARC can make troubleshooting harder, not easier.

Intermediaries that typically add ARC seals

ARC seals most often show up when mail passes through systems that regularly forward or re-mail messages, such as:

• Corporate or university email gateways that relay mail to another provider
• Mailing list managers and group distribution services
• Some hosted inbound filtering services that sit in front of the final mailbox
• Multi-tenant mailbox providers that support user-level forwarding

If you are troubleshooting forwarded newsletter deliverability, checking whether these intermediaries add ARC (and whether the chain validates) is often the quickest way to understand why DMARC failed at the final destination.

Troubleshooting forwarded newsletter deliverability with ARC results

Reading ARC headers in a delivered message

Start with a message that was forwarded and then delivered (or junked) at the final mailbox. View the full headers and look for repeated sets of:

• ARC-Authentication-Results: i=...
• ARC-Message-Signature: i=...
• ARC-Seal: i=...

Then work from the lowest instance to the highest. The highest i= is usually the last intermediary before the recipient.

What to look for quickly:

• Is the instance sequence continuous? You want i=1, then i=2, and so on, with no gaps.
• Does the latest ARC-Seal show cv=pass? A passing chain makes ARC evidence usable. If you see cv=fail, treat ARC as untrusted and focus on why the chain broke.
• Do the AAR lines show an earlier SPF/DKIM/DMARC pass? This is the “proof” that your newsletter authenticated before forwarding changed something.

If you are collecting samples for analysis, save at least one header set for each forwarding path (work to personal, list to mailbox, gateway to mailbox). Different paths can behave very differently.

Diagnosing DMARC failures with ARC evidence

Once you confirm DMARC failed at the final mailbox, ARC helps you answer the most important question: did the message ever authenticate cleanly?

Typical patterns:

• DMARC fails at the final hop, but earlier AAR shows aligned DKIM pass
  This strongly suggests forwarding modified the message after the good DKIM result was recorded. Common culprits are footers, subject prefixes, and link rewriting.

• DMARC fails at the final hop, but earlier AAR shows aligned SPF pass
  This suggests the original delivery path was authenticated, but the forwarder re-sent with a new envelope-from and different IP, causing SPF alignment to be lost downstream.

• AAR shows failures even at i=1
  That points back to your source authentication, not forwarding. In practice, this is where you re-check DKIM signing on the From domain and confirm DMARC alignment.

A useful habit is to compare the “best” hop in ARC (the earliest pass) with the final hop’s failures. The gap between them is usually where the message was altered.

Proving impact with deliverability and complaint metrics

ARC is easiest to justify when you can show that forwarded mail behaves differently after ARC-aware intermediaries are involved.

Track a few metrics separately for forwarded traffic vs direct-to-inbox traffic:

• Inbox placement vs spam placement for forwarded recipients (even a simple mailbox sampling approach is helpful).
• Bounce and rejection reasons that mention DMARC, SPF, or DKIM.
• Complaint rate and unsubscribe rate for forwarded cohorts, since false positives often show up as complaints from people who did want the mail but did not recognize it after it was modified.

If you work with larger customers (enterprises, schools, communities), ask which gateways or lists are in the path and whether they can enable ARC sealing. In many real-world cases, fixing the intermediary is faster than trying to make a newsletter “forward-proof” through DKIM alone.