The short answer is no. DKIM (DomainKeys Identified Mail) does not directly authenticate the Return-Path header. Its primary purpose is to verify that certain parts of an email, including the message body and specific headers, have not been altered in transit. The authentication of the Return-Path is handled by a different email authentication protocol, SPF (Sender Policy Framework).
However, the relationship between these headers is crucial for overall email security, especially when we bring DMARC into the picture. Let's break down how each piece works.
What does DKIM actually do?
DKIM's job is to provide a cryptographic signature for an email. When an email is sent, the sending server creates a unique digital signature. This signature is created using a private key and is attached to the email in a special header called the DKIM-Signature.
This signature header contains a tag, d=, which specifies the domain that signed the message. When a receiving server gets the email, it can use the domain from the d= tag to look up the corresponding public key in the DNS records for that domain. If the signature can be verified with the public key, it proves two things:
- Authenticity: The email was authorized by the owner of the signing domain.
- Integrity: The signed parts of the email (like the 'From' header and body) were not tampered with after the signature was applied.
Crucially, DKIM doesn't sign every header. As the official RFC 6376 specification notes, the Return-Path is one of the headers that is explicitly not supposed to be signed, as it's often added or modified by mail servers during transit.
Understanding the 'Return-Path' header
The Return-Path header, also known as the "envelope sender" or "bounce address," serves a very different purpose. It's not typically visible to the end user. Its main function is to tell receiving mail servers where to send bounce notifications if an email fails to be delivered.
This header is the key identifier used for SPF authentication. SPF works by allowing a domain owner to publish a list of IP addresses that are authorized to send email on behalf of that domain. When an email arrives, the receiving server checks the domain in the Return-Path header, fetches that domain's SPF record from DNS, and verifies if the sending IP address is on the approved list.
The role of DMARC and identifier alignment
So, we have DKIM authenticating the signing domain and SPF authenticating the Return-Path domain. This is where DMARC (Domain-based Message Authentication, Reporting, and Conformance) comes in. DMARC acts as a policy layer on top of SPF and DKIM, telling receiving servers what to do with emails that fail these checks.
A key concept in DMARC is identifier alignment. For an email to pass DMARC, it must not only pass SPF or DKIM but also be "aligned."
- SPF Alignment: The domain in the Return-Path header must match the domain in the visible 'From' header.
- DKIM Alignment: The domain in the DKIM signature's d= tag must match the domain in the visible 'From' header.
An email only needs to pass and align for either SPF or DKIM to be considered DMARC compliant. This is how the Return-Path and DKIM signature relate: they are two separate paths to achieving DMARC validation.
So, what's the final verdict?
To circle back to the original question: DKIM does not authenticate the Return-Path header. Each authentication standard has its specific target:
- SPF authenticates the server's permission to send from the domain listed in the Return-Path.
- DKIM authenticates the message's integrity and verifies it was sent by the domain listed in the d= tag of the DKIM signature.
- DMARC checks for alignment between these authenticated domains and the visible 'From' address, enforcing a policy based on the results.
While they work together under the DMARC umbrella to secure your email, their roles are distinct and separate. DKIM's focus is on message integrity, not on the address used for bounce handling.
