Summary
The question of signing the MAIL FROM (or Return-Path) address using OpenDKIM is a common point of confusion for those new to email authentication. While OpenDKIM primarily focuses on signing header fields, the MAIL FROM address is part of the SMTP envelope, distinct from the email headers. Attempting to directly sign the MAIL FROM address with OpenDKIM in the same manner as the 'From' header is generally not supported and can lead to DMARC alignment issues. Best practices dictate that DKIM signing applies to the message headers, ensuring authenticity and preventing tampering during transit.
Key findings
- Mail From vs. Header From: The MAIL FROM address is an SMTP envelope address, primarily used for bounce handling (Return-Path), whereas the 'From' header is what users typically see.
- DKIM Focus: OpenDKIM, in line with RFC specifications, is designed to sign email headers, not the SMTP envelope MAIL FROM address.
- DMARC Alignment: For DMARC to pass, the domain in the 'From' header (RFC5322.From) must align with the domain signed by DKIM, not necessarily the MAIL FROM (RFC5321.From or Return-Path) address. Learn more about DKIM alignment for DMARC compliance.
- Alternative Solutions: Some specific MTAs (Mail Transfer Agents) like PowerMTA might offer features to manipulate the signing domain, but this isn't standard OpenDKIM functionality for the MAIL FROM.
- Configuration for Headers: OpenDKIM allows you to specify which headers to sign using the SignHeaders setting in opendkim.conf, including the 'Sender' header.
Key considerations
- Adherence to RFCs: Modifying default DKIM behavior to sign the MAIL FROM directly is not aligned with RFC 4871 and could lead to unexpected deliverability issues.
- Deliverability Impact: Prioritize proper DKIM signing of the 'From' header and SPF alignment for the MAIL FROM domain to ensure high email deliverability and domain reputation.
- Configuration Complexity: Attempting to force OpenDKIM to sign in a non-standard way can introduce significant complexity and require extensive testing, often without a beneficial outcome.
- Client Requirements: If a customer requires a specific signing behavior, it is crucial to educate them on DKIM standards and the implications for deliverability, guiding them toward solutions that maintain good email hygiene.
What email marketers say
Email marketers often encounter situations where they need to manage DKIM keys for multiple domains, especially when using third-party sending services or dealing with client-specific requirements. The discussion among marketers highlights the challenges of aligning customer expectations with technical limitations and best practices for email authentication. While there's a desire for flexible signing options, the consensus leans towards adhering to standard DKIM configurations to avoid deliverability pitfalls.
Key opinions
- Client Demands: Marketers frequently face pressure from clients who may have specific, non-standard requirements for DKIM signing, such as signing with a domain they control, even if it's not the primary sending domain.
- PowerMTA Capabilities: Some platforms, like PowerMTA, are noted for their advanced configuration options that allow for more granular control over DKIM signing, including forcing the 'd=' domain in the signature.
- OpenDKIM Limitations: OpenDKIM is perceived as less flexible than some commercial MTAs when it comes to unconventional signing scenarios, particularly concerning the MAIL FROM address.
- Importance of 'From' Header: The 'From' header (RFC5322.From) is recognized as the critical component for DKIM alignment, especially concerning DMARC.
Key considerations
- Educating Clients: It's essential to explain the technical realities of DKIM and DMARC alignment to clients to ensure they understand why certain signing configurations are necessary for deliverability.
- Avoiding DMARC Failures: While some configurations might technically be possible with specific MTAs, they often lead to DMARC failures if not properly aligned, negatively impacting inbox placement. This is particularly important for marketers dealing with DMARC failures due to lack of DKIM signing.
- Simplicity vs. Customization: For OpenDKIM users, adopting a simpler strategy like signing all domains with the same key can ease management, even if it means more keys to rotate, as described by some experts for postfix configurations.
- Subdomain Strategy: Consider if setting up DKIM on a subdomain is a viable strategy for handling different sending needs while maintaining alignment.
Marketer view
An email marketer from Email Geeks indicates a strong desire to sign the sender address via OpenDKIM, specifically the MAIL FROM, noting that PowerMTA allows this while OpenDKIM's capability is unknown to them. They are currently only able to sign the 'From' header.
Marketer view
An email marketer from a Linux forum explains the process of generating both private and public keys for each domain intended for mail signing. They highlight the importance of securely storing the private key on the server and publishing the public key in the domain's DNS as a TXT record for verification by recipients.
What the experts say
Experts in email deliverability emphasize the distinction between the SMTP MAIL FROM (Return-Path) and the 'From' header when it comes to DKIM signing. They highlight that standard DKIM protocols do not sign the MAIL FROM address directly, as it's an envelope address. Attempts to force this behavior, while possible with certain MTAs, often lead to DMARC alignment issues and are not considered best practice. The focus remains on correctly signing the 'From' header and ensuring proper alignment for robust email authentication.
Key opinions
- RFC Compliance: Experts concur that RFC 4871 explicitly states the MAIL FROM/Return-Path address should not be part of the DKIM signature.
- SMTP Transaction vs. Header: The MAIL FROM address is transmitted during the SMTP transaction and only appears as the Return-Path header upon recipient server receipt, making it unsuitable for DKIM signing at the sender's end.
- Direct Header Signing: OpenDKIM's primary function is to sign actual email headers such as 'From', 'To', 'Subject', and 'Sender', using the SignHeaders setting.
- Alignment Risks: Attempts to override standard signing behavior, even with MTAs like PowerMTA that offer such options, will likely lead to DMARC failures due to misaligned domains.
- Multiple DKIM Keys: OpenDKIM can manage multiple DKIM keys, automatically selecting the appropriate key based on the 'From' address, simplifying setup for multiple domains on a single server, as detailed by Al Iverson on Spam Resource.
Key considerations
- Best Practices: Sticking to standard DKIM signing of message headers is crucial for reliable email deliverability and authentication. Deviating from these practices creates unnecessary complexity and risk.
- Configuration Complexity: Implementing non-standard signing requires significant effort in configuration and testing, often without yielding positive results for deliverability. This also affects how different mail from headers and domains are handled.
- SPF and DKIM Alignment: Even with dual signing or specific MTA configurations, ensuring both SPF and DKIM align with the 'From' header domain remains paramount for DMARC to pass. Our guide to DMARC, SPF, and DKIM provides a good overview.
- System Choice: The choice of MTA (e.g., Postfix vs. PowerMTA) impacts the flexibility of DKIM configurations, with OpenDKIM integrating as a milter in systems like Postfix.
Expert view
An expert from Email Geeks clarifies that the MAIL FROM address, also known as the Return-Path address, should not be included in the DKIM signature according to RFC 4871. This fundamental principle ensures the integrity of the signature while adhering to established standards.
Expert view
An expert from Spam Resource recommends signing every domain with the same DKIM key on an OpenDKIM installation, despite the need to rotate multiple keys simultaneously. This approach simplifies the configuration process for managing mailing from multiple domains on a single Linux server using Postfix.
What the documentation says
Official documentation and RFCs provide the foundational understanding for how DKIM is designed to function. They clearly differentiate between the various email addresses and headers, stipulating which elements are subject to cryptographic signing. This technical guidance is crucial for proper implementation and for ensuring that email authentication mechanisms work as intended across the internet.
Key findings
- RFC 4871 Directives: According to RFC 4871, the MAIL FROM (or Return-Path) address is specifically excluded from the cryptographic hash used for DKIM signatures, as it is part of the SMTP envelope and not a message header.
- Header Signing Purpose: DKIM is primarily designed to verify the integrity and authenticity of email headers and, optionally, the message body, protecting them from tampering during transit.
- OpenDKIM Configuration: OpenDKIM configurations typically involve specifying a list of headers to be signed using the SignHeaders directive in its configuration file, reinforcing that it operates on headers.
- Milter Integration: OpenDKIM integrates with MTAs like Postfix via a milter interface, allowing it to process and sign messages as they pass through, but still within the context of message headers.
Key considerations
- Standard Compliance: Adhering to RFCs and standard OpenDKIM configurations is critical for ensuring that DKIM signatures are correctly generated and validated by receiving mail servers worldwide.
- DMARC Implications: DMARC requires alignment between the 'From' header and either the SPF domain (MAIL FROM) or the DKIM signing domain. Deviating from standard DKIM practices for the 'From' header will almost certainly lead to DMARC failures.
- DomainKeys vs. DKIM: While DomainKeys (the predecessor to DKIM) had some different nuances, the current standard (DKIM) focuses on header signing. Understanding DomainKeys and their relevance can provide historical context but modern implementations should follow DKIM.
Technical article
RFC 4871, Section 3.7 states that the 'Return-Path' header field, which is derived from the MAIL FROM address, is specifically excluded from the list of headers that must be signed. This ensures that changes to the envelope address by intermediate mail servers do not invalidate the DKIM signature.
Technical article
The OpenDKIM documentation outlines the 'SignHeaders' configuration option, which specifies the list of message headers that should be included in the DKIM signature. This emphasizes that OpenDKIM operates on the email's header section.
Related resources
8 resources
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Related pages
How to use DKIM to sign emails with different header from domains to comply with DMARC?
When changing subdomains for email, is it better to change the 5321.from or 5322.from header for deliverability, and how does DKIM alignment affect this?
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