It's a great question, and one that highlights how technical terms can sometimes have multiple meanings across different industries. When we talk about an 'arc-source' tag in the context of email, we're stepping into the world of email authentication, specifically a protocol called Authenticated Received Chain, or ARC. However, as you may have discovered, searching for this term can bring up results about electrical safety, cloud computing, and more, which can be quite confusing. Let's clear this up and focus on what ARC means for your emails.
What is Authenticated Received Chain (ARC)?
ARC is an email authentication protocol designed to solve a common problem that affects email deliverability: broken authentication. Standard authentication methods like SPF and DKIM work wonderfully for direct email delivery. However, when an email passes through an intermediary, like a mailing list or a forwarding service, these checks can fail. The intermediary's server is not authorized in the sender's SPF record, and modifications made by the mailing list (like adding a subject prefix or an unsubscribe footer) can invalidate the DKIM signature.
This is where ARC comes in. It allows an intermediary mail server to see the initial authentication results (before it arrived at the intermediary). The intermediary then signs the email with its own ARC signature, effectively creating a chain of custody. When the final receiving server gets the email, it can look at this chain of ARC headers. If it trusts the intermediaries in the chain, it can use the original, passing authentication results to inform its filtering decisions, even if the final SPF and DKIM checks fail.
The components of ARC
ARC works by adding three new headers to an email at each 'hop' through a trusted intermediary. These headers are added in a specific sequence:
- ARC-Authentication-Results (AAR): This header is a snapshot of the authentication results (SPF, DKIM, DMARC) as the intermediary server saw them. It includes an instance number (e.g., i=1) to keep the headers in order.
- ARC-Message-Signature (AMS): This is a DKIM-like signature that covers the entire message content, including headers that may have been modified by the intermediary. This signature essentially says, "This is what the message looked like when I, the intermediary, processed it."
- ARC-Seal (AS): This is the final header, and it's a signature that covers the previous ARC headers (the AAR and AMS). It validates the integrity of the ARC information itself, sealing the results from that particular hop. It contains a chain validation status tag (cv) which will be either 'pass' or 'fail'.
So, what about the 'arc-source' tag?
This brings us back to your original question. Based on the official ARC specification (RFC 8617), there is no standard tag called 'arc-source'. The specification defines tags like i (instance), d (domain), s (selector), and cv (chain validation status), but 'arc-source' is not among them. It is possible that a specific email service provider or software has implemented a custom header or tag with this name to add extra information, but it is not part of the universal standard that mail servers use to process ARC.
The source of confusion: 'arc' in other contexts
The confusion is understandable because the terms "ARC" and "arc source" are used in completely unrelated fields. For instance, in electrical engineering, an "arc source" is a critical safety concept.
In supply chain and retail, ARC is an important certification for RFID tags, which has nothing to do with email.
And in cloud computing, particularly Microsoft Azure, "ARC" refers to a service for managing hybrid cloud resources, where "tags" are used for metadata.
Final thoughts
In the world of email, understanding the correct protocols is key to ensuring your messages land in the inbox. While the term 'arc-source' might not be a standard part of the email lexicon, understanding the Authenticated Received Chain (ARC) is vital. If you send emails that are likely to be forwarded or distributed via mailing lists, ensuring your systems and your partners' systems support ARC can make a significant difference in preserving your sender reputation and improving deliverability.
