--- title: "ARC Email Authentication: The Layer After SPF and DMARC" description: "ARC email authentication explained - how the Authenticated Received Chain preserves SPF and DKIM when mail is forwarded, and why cold senders should care." date: "2026-06-14" tags: "email-deliverability, email-authentication, dmarc, cold-email, dkim" readTime: "12 min read" author: "FirstSales Team" slug: "arc-email-authentication" canonical: "https://firstsales.io/blog/arc-email-authentication/" --- **TL;DR:** ARC email authentication, short for Authenticated Received Chain, fixes a problem SPF and DKIM cannot solve on their own: when a message is forwarded or passes through a mailing list, the forwarding server often breaks the original SPF and DKIM checks. ARC lets each intermediate server cryptographically record what the authentication results were when it received the message, so the final inbox can see the original verdict even after forwarding altered the message. For cold senders whose mail gets routed through gateways, security appliances, or auto-forwarding rules, ARC can be the difference between a delivered message and a silent failure. This guide explains how it works and why it matters. ## Table of Contents - [Why SPF and DKIM Break on Forwarding](#why-spf-and-dkim-break-on-forwarding) - [What ARC Actually Is](#what-arc-actually-is) - [The Three ARC Headers Explained](#the-three-arc-headers-explained) - [How the Chain Preserves the Original Verdict](#how-the-chain-preserves-the-original-verdict) - [Where Cold Email Gets Forwarded](#where-cold-email-gets-forwarded) - [Why ARC Matters for Cold Senders](#why-arc-matters-for-cold-senders) - [What You Can and Cannot Control About ARC](#what-you-can-and-cannot-control-about-arc) - [ARC in the Wider Authentication Stack](#arc-in-the-wider-authentication-stack) - [Reading ARC Results in Message Headers](#reading-arc-results-in-message-headers) - [Common Misconceptions About ARC](#common-misconceptions-about-arc) - [FAQs](#faqs) - [Conclusion](#conclusion) --- ## Why SPF and DKIM Break on Forwarding To understand why ARC email authentication exists, you have to understand a specific failure that happens millions of times a day. A message passes SPF and DKIM when it leaves your server. Then it gets forwarded. After the forward, the same message fails the same checks at the next inbox. Nothing about your setup was wrong. Forwarding broke it. Here is why. SPF checks whether the sending IP is authorized to send for the envelope-sender domain. When a forwarding server relays your message onward, the message now arrives at the next inbox from the forwarder's IP, not your IP. The next inbox looks up your SPF record, sees the forwarder's IP, and finds it is not on your authorized list. SPF fails. Your record was correct. The forward just changed the IP that delivered the final hop. DKIM is more robust because its cryptographic signature travels with the message, but it breaks too. DKIM signs specific headers and the body. If the forwarding server modifies anything covered by the signature - rewrites the subject line by prepending "Fwd:", appends a mailing-list footer, adds a banner, changes the encoding - the signature no longer validates. Many forwarders and mailing lists do exactly this. The signature breaks not because anyone tampered maliciously, but because the legitimate forwarder edited the message. So you get a message that genuinely passed authentication at origin, failed it after forwarding, and now looks unauthenticated to the final receiver. Under a strict DMARC policy, that message can be quarantined or rejected. The original sender did nothing wrong, but the mail still fails. This is the gap ARC was designed to close. It is worth pausing on how invisible this failure is to the sender. You never see it. Your sending logs show the message accepted at the first hop, which it was. Your own authentication checks pass, because at origin they do pass. There is no bounce that points at forwarding, because the message was accepted and then forwarded and only failed at a hop you have no visibility into. From your seat, the campaign looks fine. The prospect just never replied, and you have no way to know whether they ignored you or never received the mail. This invisibility is precisely why ARC is worth understanding even though you cannot configure it: it is the mechanism that determines the fate of a whole category of your mail that you would otherwise have no insight into at all. Without knowing ARC exists, a sender confronted with patchy delivery to enterprise prospects has no framework for even guessing the cause, and will usually misdiagnose it as a copy or targeting problem when it is actually a routing-and-authentication problem happening two hops downstream. --- ## What ARC Actually Is ARC, the Authenticated Received Chain, is an email authentication standard that lets each server handling a message record the authentication results it observed, and cryptographically seal that record so a later server can trust it. Think of it as a tamper-evident chain of custody for authentication verdicts. The core idea is simple. When an intermediate server (a forwarder, a mailing list, a security gateway) receives a message that passed SPF and DKIM, it can attach an ARC seal that says, in effect: "When I received this message, SPF passed and DKIM passed. I am vouching for that, and here is my cryptographic signature proving I am the one saying it." When the message reaches the final inbox, even though SPF and DKIM now fail because of the forwarding, the inbox can look at the ARC chain, see that a trusted intermediary recorded a passing verdict before it modified the message, and choose to honor that. ARC does not replace SPF, DKIM, or DMARC. It sits alongside them as a way to carry the original authentication verdict across hops that would otherwise destroy it. It is purely additive. It gives a receiving server one more piece of evidence to make a delivery decision when the primary checks fail due to legitimate forwarding. The critical thing to understand is that ARC is built on trust. A receiving inbox only honors an ARC chain if it trusts the servers that sealed it. A spammer can add their own ARC seal claiming the message passed, but if the receiver does not trust that sealer, the seal is worthless. ARC works because the major mailbox providers maintain reputation on ARC sealers, just as they do on sending domains. This trust model is what stops ARC from being a forger's tool. On its face, a header that says "trust me, this passed" sounds trivially abusable. The reason it is not is that the claim is only as good as the entity making it, and the entity making it has its own reputation to protect. A well-behaved corporate gateway or mailing-list operator that consistently seals honestly accumulates a good reputation as a sealer, and its seals get honored. An entity that seals dishonestly - claiming passes for mail that did not pass - gets distrusted, and its seals get ignored, which makes the dishonest seal pointless. The economics favor honesty, which is the same reason the broader email reputation system works at all. So when you read that "ARC preserves the verdict," the unstated condition is "as vouched for by a sealer the receiver has independent reason to trust." Strip out that trust layer and ARC would be meaningless; it is the trust, not the headers, that does the real work. --- ## The Three ARC Headers Explained ARC works through three headers that each intermediate server adds. Understanding them demystifies the whole mechanism. **ARC-Authentication-Results (AAR).** This header records the authentication results - SPF, DKIM, and DMARC verdicts - exactly as the intermediate server observed them when it received the message. It is a snapshot of the verdict at that hop. If the server received a message that passed DKIM and SPF, the AAR records that. **ARC-Message-Signature (AMS).** This is a signature over the message contents at the moment the intermediate server received it, similar to a DKIM signature. It captures the state of the message before this server made any modifications. This is what lets a later server verify that the recorded verdict actually corresponds to the message as it existed at that hop. **ARC-Seal (AS).** This signs the ARC headers themselves, including all previous ARC header sets in the chain. The seal is what makes the chain tamper-evident. Each new intermediary's seal covers everything before it, so you cannot rip out or alter an earlier link without breaking the chain. Together these form one "ARC set" per hop, each tagged with an instance number (`i=1`, `i=2`, and so on). A message forwarded twice has two ARC sets. The final receiver validates the chain by walking it: checking that each seal is intact, that the chain has not been broken, and that it trusts the sealers. The instance numbering matters. It establishes order. The receiver can see this message went through hop one, then hop two, and that the verdict at hop one was "pass" before any modification happened. That ordered, sealed record is the whole value of ARC. --- ## How the Chain Preserves the Original Verdict Walk through a concrete forwarding scenario to see ARC do its job. You send a cold email. It leaves your authenticated domain with valid SPF and DKIM. The first recipient's mail system is a corporate gateway that scans the message, then forwards it to the actual user's mailbox, which happens to be hosted on a different provider. Along the way, the gateway appends a "scanned by security" footer to the body. **At the gateway:** The gateway receives your message and runs authentication. SPF passes, DKIM passes, DMARC passes. The gateway is ARC-aware, so it adds an ARC set. The AAR header records "spf=pass, dkim=pass, dmarc=pass." The AMS signs the message as received. The AS seals the chain. Then the gateway appends its footer and forwards the message. **At the final inbox:** The message arrives from the gateway's IP, with a body that now has a footer. The inbox runs its own SPF check - it fails, because the IP is the gateway's, not yours. It runs DKIM - it fails, because the footer changed the body. Under a strict DMARC policy, this message is now a candidate for the spam folder or outright rejection. **Then ARC kicks in.** The inbox examines the ARC chain. It finds the gateway's ARC set, which records that SPF, DKIM, and DMARC all passed when the gateway received the message. The inbox checks the ARC seal - intact. It checks whether it trusts this gateway as an ARC sealer. If it does, it can override the local SPF and DKIM failures and deliver the message, because a trusted intermediary vouched that the message was authentic before forwarding altered it. That is the entire mechanism. ARC does not magically un-break SPF and DKIM. It preserves the evidence that they passed earlier, sealed by someone the receiver trusts, so the forwarding failure does not condemn a legitimate message. Without ARC, that perfectly valid email could land in spam purely because it was forwarded. --- ## Where Cold Email Gets Forwarded You might assume forwarding is rare for cold outbound. It is more common than people think, and the paths are often invisible to the sender. **Corporate mail gateways and security appliances.** Many companies route inbound mail through a security gateway (Proofpoint, Mimecast, Barracuda, and similar) before it reaches user mailboxes. To the receiving user's mailbox, the message arrives from the gateway, not from you. This is a forwarding hop in everything but name, and it can break SPF and DKIM exactly as described. **Auto-forwarding rules.** A surprising number of professionals forward their work email to a personal account, or route certain senders to an assistant. The prospect you emailed may have a rule that silently forwards your message to another mailbox where the actual reading and replying happens. That forward can break your authentication at the destination. **Distribution lists and aliases.** You email `sales@company.com`, which is actually an alias that fans out to four people. Each delivery is effectively a forward. Aliases and small internal distribution lists are everywhere in B2B, and your cold email hits them constantly. **Mailing lists and group addresses.** Some role addresses are backed by mailing-list software that rewrites headers and adds footers - the classic DKIM-breaking behavior. The point is that you do not control whether your mail gets forwarded after it arrives. The recipient's infrastructure decides. A meaningful slice of your cold email passes through at least one of these hops, and at each hop, ARC support on the intermediary determines whether your original passing verdict survives. This is part of why two technically-identical cold emails can have different deliverability: one hit a direct mailbox, the other hit a gateway-forwarded mailbox where authentication broke and ARC was the only thing that could save it. --- ## Why ARC Matters for Cold Senders Here is the honest framing, because the cold-email industry tends to oversell every authentication acronym. ARC is not a setting you flip to boost deliverability. It is mostly a mechanism that runs on the receiving and forwarding side. But it matters for cold senders in a few real ways. **It explains a category of mysterious failures.** If you have ever seen a campaign where some recipients clearly never got your mail despite valid authentication on your end, forwarding-induced authentication breakage is a likely culprit, and ARC support at the intermediary is what determines whether those messages survive. Knowing this stops you from chasing the wrong fix - you are not going to solve a forwarding break by rewriting your subject line. **It rewards a clean DMARC posture.** ARC only helps when your message genuinely passed authentication at origin. If your SPF, DKIM, and DMARC are not solid to begin with, there is no passing verdict for an ARC sealer to preserve. ARC is downstream of getting your own [SPF, DKIM, and DMARC setup](/blog/spf-dkim-dmarc-setup-2026/) right. The cleaner your origin authentication, the more ARC can do for you when forwarding happens. **It interacts with your DMARC enforcement level.** If you are running a strict DMARC policy, you depend more heavily on ARC to rescue legitimately-forwarded mail, because strict policies are exactly what condemn the forwarding-broken messages. This is one more reason to stage your enforcement carefully, the way a sensible [DMARC quarantine policy](/blog/dmarc-quarantine-policy/) rollout does, rather than jumping to reject and silently losing forwarded mail. **It is part of why ESP and infrastructure choice matters.** When you send through reputable infrastructure that participates correctly in the ARC ecosystem and maintains good standing as a sealer, your forwarded mail fares better than when you send through abused, low-trust infrastructure. This ties back to the broader question of why your [cold emails land in spam](/blog/why-cold-emails-land-in-spam/) - sometimes the answer is a forwarding hop you never saw, on infrastructure with weak standing. So ARC is not a lever you pull. It is a reason to keep your origin authentication impeccable and to choose infrastructure that plays well in the ecosystem. It rewards senders who do the fundamentals and quietly punishes those who do not, in a category of failures most senders never even diagnose. --- ## What You Can and Cannot Control About ARC This section keeps you out of the trap of trying to "set up ARC" on your sending domain, which is mostly not a thing you do as a sender. **What you cannot control:** - Whether your recipient's mail system forwards your message. - Whether the forwarding intermediary supports ARC and seals correctly. - Whether the final receiving inbox trusts that intermediary's ARC seals. - The ARC reputation of corporate gateways your mail passes through. These are receiver-side and intermediary-side behaviors. As a sender, they are out of your hands. Accept that and stop trying to configure your way around them. **What you can control:** - **Pristine origin authentication.** Your SPF, DKIM, and DMARC must pass cleanly at send time. ARC can only preserve a verdict that existed. Give it a passing verdict to preserve. - **DKIM alignment that survives.** Use your own domain's DKIM key with proper alignment so the original signature is as strong as possible before forwarding. - **Reputable sending infrastructure.** Send through infrastructure with good standing so that when intermediaries and receivers evaluate your mail, the surrounding signals favor delivery. - **A staged DMARC policy.** Do not jump to strict enforcement before you understand how much of your mail gets forwarded. A measured rollout lets you watch for forwarding-related failures in your aggregate reports before they cost you delivery, and pairing that with ongoing [deliverability monitoring](/blog/email-deliverability-monitoring/) is how you catch the damage while it is still small. - **Message hygiene.** Cleaner, simpler messages (less likely to be heavily rewritten, fewer fragile elements) survive forwarding modification better, which keeps more of your DKIM intact even on hops without ARC. The mental model: you cannot install ARC, but you can be the kind of sender ARC helps. ARC helps senders whose mail truly passed authentication and who run on trustworthy infrastructure. Be that sender. --- ## ARC in the Wider Authentication Stack It helps to place ARC in the full picture so you understand the order of operations and what depends on what. | Layer | What it does | Who controls it | Breaks on forwarding? | |---|---|---|---| | SPF | Authorizes sending IPs for a domain | Sender (DNS) | Yes, IP changes at forward | | DKIM | Cryptographically signs the message | Sender (DNS + ESP) | Yes, if body or signed headers change | | DMARC | Sets policy when SPF and DKIM fail | Sender (DNS) | It is the policy that condemns broken mail | | ARC | Preserves the original verdict across hops | Intermediaries and receivers | No, that is its entire purpose | Read top to bottom, this is the dependency chain. SPF and DKIM establish authentication at origin. DMARC decides what happens when they fail. ARC exists specifically to stop DMARC from punishing mail that only failed because of legitimate forwarding. ARC is the patch over the seam where forwarding meets strict DMARC enforcement. The practical sequencing for a sender is therefore: get SPF and DKIM correct, set up DMARC and stage your enforcement carefully, and understand that ARC is the thing working in the background to rescue your legitimately-forwarded mail - which only works if the first three layers are solid. You do not start with ARC. You earn ARC's help by getting the foundation right. This is also why ARC is genuinely an advanced topic, not a starter one. A sender who has not yet published clean DMARC has no business worrying about ARC. A sender running strict DMARC at scale across forwarded recipients absolutely should understand it, because for them, ARC is the difference between strict enforcement that protects them and strict enforcement that silently eats their forwarded mail. --- ## Reading ARC Results in Message Headers You cannot configure ARC as a sender, but you can read it, and reading it is genuinely useful for diagnosing the forwarding-related failures that otherwise look like random deliverability noise. The information lives in the message headers of mail that passed through an ARC-aware hop. When you open a received message and view its original source - in Gmail, the "Show original" option; in Outlook, viewing message properties or internet headers - you see the full header block. On a message that traveled through a forwarding hop, you will find the three ARC headers described earlier: `ARC-Authentication-Results`, `ARC-Message-Signature`, and `ARC-Seal`, each tagged with an instance number. You may also see an `Authentication-Results` header that mentions `arc=pass` or `arc=none`, which tells you whether the receiving server found and validated an ARC chain. What you learn from reading these: - **Whether the message was forwarded at all.** The presence of ARC headers is a strong signal the message passed through an intermediary that added them. No ARC headers usually means a direct delivery. - **What the original verdict was.** The `ARC-Authentication-Results` header records the SPF, DKIM, and DMARC verdict the intermediary observed. If it says the original passed, you know your authentication was clean at origin and any later failure was forwarding-induced. - **Whether the chain validated.** An `arc=pass` in the receiver's results means the chain was intact and trusted. An `arc=fail` or absence means the receiver did not honor the chain, which helps explain why a forwarded message may have landed in spam instead of counting toward your [inbox placement rate](/blog/inbox-placement-rate/). For a cold sender, this is diagnostic gold in the specific case where you suspect forwarding is hurting you. If a prospect at a company behind a known security gateway never seems to get your mail, and you can get a copy of a message that did arrive (or test against a mailbox behind a similar gateway), reading the ARC headers tells you whether the gateway is sealing correctly and whether the final inbox honored the seal. That moves you from guessing to knowing. The practical workflow: when you see inconsistent delivery to recipients you know sit behind corporate mail infrastructure, do a controlled test send to a seed mailbox behind comparable infrastructure, then read the headers. You are checking three things - did your origin authentication pass, did the intermediary add a valid ARC seal, and did the final receiver honor it. Wherever that chain breaks tells you where the problem is. Often it confirms that your side is clean and the issue is downstream, which at least stops you from wasting time rewriting copy to fix an infrastructure-routing problem. --- ## Common Misconceptions About ARC ARC attracts a particular kind of confusion because it sounds like another authentication record you should "set up." Clearing these misconceptions saves you from wasted effort and wrong conclusions. **Misconception: I need to add an ARC record to my DNS.** No. Unlike SPF, DKIM, and DMARC, ARC is not a DNS record you publish as a sender. It is added by intermediaries that forward your mail and validated by receivers. There is nothing to publish on your sending domain. Time spent looking for "how to set up ARC for my domain" is time wasted, because that is not how ARC works. **Misconception: ARC boosts my deliverability.** ARC does not raise your reputation or get you into more inboxes by itself. It prevents a specific failure - legitimately-forwarded mail being condemned by strict DMARC. If none of your mail gets forwarded, ARC does essentially nothing for you. It is a patch over a seam, not a booster. **Misconception: ARC fixes broken SPF and DKIM.** ARC does not repair the broken checks. After forwarding, SPF and DKIM still fail at the final hop. ARC simply preserves the record that they passed earlier. The receiver still sees the current failures; it just has additional sealed evidence of the prior success to weigh against them. **Misconception: any ARC seal will rescue my mail.** Only seals from intermediaries the receiver trusts carry weight. A spammer can stamp their own ARC seal claiming the message passed, but if the receiver does not trust that sealer, the seal is ignored. ARC works because major providers maintain reputation on sealers. An untrusted seal is no seal at all. **Misconception: ARC means I can relax my own authentication.** The opposite is true. ARC can only preserve a verdict that existed. If your origin SPF, DKIM, and DMARC do not pass cleanly, there is no passing verdict for any intermediary to seal. ARC raises the value of getting your own authentication impeccable, because impeccable origin authentication is the only thing that gives ARC something worth preserving. The unifying correction across all of these: ARC is downstream of your work, not a piece of your work. You influence it indirectly by being a clean, well-authenticated sender on reputable infrastructure. You do not touch it directly. Internalize that and you will stop trying to configure something that is not yours to configure, and instead focus on the origin authentication and infrastructure choices that actually determine how much ARC can help you. --- ## FAQs ### Do I need to set up ARC on my sending domain? Not in the way you set up SPF, DKIM, or DMARC. ARC is primarily added by intermediate servers that forward your mail, and honored by receiving servers. As a sender, you do not publish an ARC DNS record. What you do is keep your origin SPF, DKIM, and DMARC clean so there is a passing verdict for ARC-aware intermediaries to preserve when your mail gets forwarded. ### Does ARC improve cold email deliverability directly? Not directly. ARC does not boost your reputation or get you into more inboxes by itself. It prevents a specific category of failure - legitimately-forwarded mail being rejected because forwarding broke SPF and DKIM. If a meaningful share of your prospects sit behind gateways or auto-forward rules, ARC working correctly upstream protects those deliveries. But it only helps mail that genuinely passed authentication at origin. ### What breaks SPF when an email is forwarded? The sending IP changes. SPF checks whether the IP delivering the message is authorized for the sender's domain. When a server forwards your message, the final inbox sees the forwarder's IP, not yours. Since the forwarder's IP is not on your SPF record, SPF fails at the final hop even though your record is correct. ### Why does forwarding break DKIM signatures? DKIM signs specific headers and the message body. Many forwarders and mailing lists modify the message - adding footers, rewriting subjects, changing encoding. Any change to a signed part invalidates the signature. The message was not maliciously tampered with; a legitimate forwarder simply edited it, and that edit breaks the cryptographic signature. ### Is ARC related to DMARC? Yes. ARC exists specifically to help DMARC make better decisions on forwarded mail. Under a strict DMARC policy, a forwarded message that lost its SPF and DKIM would be quarantined or rejected. ARC lets the receiver see that the message passed authentication before forwarding altered it, so a trusted ARC chain can rescue the message from DMARC enforcement. ### Should cold senders running strict DMARC worry about ARC? Yes, more than most. Strict DMARC enforcement is exactly what condemns forwarding-broken mail. If you run a strict policy and a portion of your recipients are behind forwarding hops, ARC support upstream is what keeps that mail deliverable. This is a strong reason to stage your DMARC rollout carefully and to send through reputable infrastructure rather than rushing to strict enforcement. --- ## Conclusion ARC email authentication is the layer that handles a problem SPF, DKIM, and DMARC cannot solve on their own: the silent breakage that happens when legitimate mail gets forwarded. Forwarding changes the sending IP and often modifies the message, so SPF and DKIM fail at the final hop even when your setup is perfect. ARC preserves the original passing verdict, sealed by a trusted intermediary, so the receiving inbox can deliver mail that only failed because it was forwarded. For cold senders, the takeaway is not to "configure ARC" - it is mostly a receiver-side and intermediary-side mechanism. The takeaway is to be the sender ARC helps: keep your origin SPF, DKIM, and DMARC impeccable, send through reputable infrastructure, and stage your DMARC enforcement so you do not silently lose forwarded mail. ARC rewards the senders who do the fundamentals. Doing the fundamentals consistently across every send is where most teams slip, especially at volume. [FirstSales](https://firstsales.io) keeps the quality high at the message level: the AI drafts a personalized cold email for each prospect, you review and approve every draft, then it sends - so the mail leaving your domain is clean, authenticated, and worth preserving when it gets forwarded. Start for $1 and keep your sending foundation solid from the first campaign.