The short answer is yes, but indirectly. Authenticated Received Chain (ARC) is not a protocol designed to actively identify and block spam. Instead, its primary function is to solve a specific problem with email forwarding that can cause legitimate emails to be incorrectly flagged as spam. By fixing this, ARC allows spam filtering systems to be more effective.
Essentially, ARC preserves email authentication results (like SPF and DKIM) as an email travels through intermediaries, such as mailing lists or forwarding services. This helps the final recipient's mail server understand that an email was legitimate at its origin, even if the forwarding process broke the original authentication.
What is ARC and why does it exist?
Standard email authentication protocols, SPF and DKIM, work well for emails sent directly from a sender to a recipient. DMARC uses the results of these checks to tell the receiving server whether to accept, quarantine, or reject a message. The system breaks down, however, with indirect mailflows. When an email is sent to a mailing list, the mailing list server receives it and then forwards it to all subscribers. This forwarding act often invalidates the original authentication.
For example, the forwarding server's IP address won't match the sender's SPF record, causing an SPF failure. The mailing list might also add a footer to the email, which alters the email's body and breaks the DKIM signature. The final recipient's server sees these failures and may junk or reject the message, even though it was perfectly valid when it was first sent.
ARC solves this by creating a chain of custody. As Bento explains, ARC allows a server to see previous authentication results. When a forwarding server receives a successfully authenticated email, it adds a new ARC header that cryptographically signs the original authentication results. It's like putting the original, validated results in a sealed envelope for the next server in the chain to inspect.
How ARC indirectly helps combat spam
ARC's main contribution to fighting spam is its ability to reduce false positives. A false positive is when a legitimate email is incorrectly identified as spam. By helping legitimate forwarded emails get delivered correctly, ARC makes the job of a spam filter easier.
When a receiving mail server can trust that forwarded emails are not being misidentified, it can be more confident in its decisions. It can apply stricter rules to emails that fail authentication and do not have a valid ARC chain, without the risk of blocking important messages. This leads to several key benefits:
- Improved deliverability: ARC's primary goal is to prevent legitimate emails from being marked as spam or rejected, boosting the reliability of indirect mailflows.
- Maintained trust: It preserves the trust established by the original sender's SPF, DKIM, and DMARC records.
- Better filtering: With fewer false positives to worry about, spam filters can be more aggressive in blocking messages that are genuinely suspicious.
The relationship between ARC, SPF, DKIM, and DMARC
ARC is not a standalone protocol or a replacement for existing authentication standards. It is a layer built on top of the existing framework. For ARC to work, a sender must first have SPF, DKIM, and DMARC configured correctly.
The process works in sequence. An email is sent and validated with SPF and DKIM. The DMARC policy is checked. If that email is then passed to a forwarding service that supports ARC, that service validates the original authentication and adds the ARC-Seal header. When the email reaches its final destination, the receiving server may see that the direct SPF and DKIM checks fail. However, it can then check the ARC chain. If the ARC chain is valid and shows the email passed authentication initially, the server can choose to trust and deliver the message.
In conclusion, while ARC's direct purpose is to ensure deliverability for forwarded mail, it plays a valuable supporting role in the fight against spam. By providing a clear, verifiable history of an email's authentication, it allows email systems to make smarter, more confident filtering decisions. This reduces the chance of good mail ending up in the spam folder and enables a more aggressive stance against truly malicious messages.
