How do ESPs collect Yahoo FBL data using double DKIM signing?
Michael Ko
Co-founder & CEO, Suped
Published 27 Jul 2025
Updated 16 Aug 2025
9 min read
Yahoo's Complaint Feedback Loop (CFL), often simply called an FBL (Feedback Loop), is a critical mechanism for senders to understand how recipients perceive their emails. When a user marks an email as spam in their Yahoo mailbox, this system sends a report back to the sender, providing valuable insight into recipient engagement and potential issues. For email service providers (ESPs), collecting this data is essential for maintaining strong sender reputation and ensuring high email deliverability. Without access to FBL data, ESPs would be largely unaware of recipient complaints, which are a strong signal of unwanted mail.
The challenge for many ESPs lies in effectively collecting this FBL data from Yahoo, especially when sending on behalf of numerous clients. Yahoo's FBL program is unique because it's based on the DKIM (DomainKeys Identified Mail) signature within the email header. This means the domain used in the DKIM signature is the one that needs to be enrolled in Yahoo's CFL program to receive complaint reports. To streamline this process and ensure all client mail is covered, many ESPs employ a technique known as double DKIM signing.
Yahoo's Complaint Feedback Loop is a domain-based system, meaning it primarily relies on the DKIM (DomainKeys Identified Mail) signature attached to your emails. Unlike some other FBLs that might use IP addresses for reporting, Yahoo uses the 'd=' tag (the signing domain) in the DKIM signature to identify the entity responsible for the email. If that specific domain is enrolled in their CFL program, Yahoo sends an Abuse Reporting Format (ARF) report to the designated address whenever a recipient complains about an email signed with that DKIM domain.
This domain-centric approach places a significant emphasis on proper DKIM configuration. For an ESP, this implies that every domain whose reputation they wish to monitor via Yahoo's FBL needs to be individually enrolled. Managing thousands or even millions of client domains for FBL enrollment can be a monumental task, leading many ESPs to seek more scalable solutions. Understanding how these feedback loops function is crucial for any email service provider.
The ARF reports provided by Yahoo are invaluable for deliverability. They contain details about the complained-about email, such as the timestamp, the recipient's email address (often obfuscated for privacy), and the original message headers. Analyzing this data allows ESPs to identify problematic campaigns, list quality issues, and specific clients or sending practices that are generating high complaint rates, which are crucial for improving overall sender performance and avoiding blocklists (or blacklists).
One key aspect of Yahoo's FBL is its reliance solely on DKIM-signed emails. This is highlighted in Yahoo's own documentation on their sender hub. This reliance means that emails not properly signed with DKIM will not generate FBL reports, even if recipients mark them as spam, limiting the visibility for senders.
Understanding double DKIM signing
Double DKIM signing refers to the practice where an email is signed with two different DKIM signatures: one from the client's domain (the From domain) and another from the ESP's own domain. This means the email header will contain two distinct DKIM-Signature fields, each referencing a different signing domain. The purpose of this technique for ESPs is often to facilitate FBL data collection, among other benefits related to deliverability and sender reputation. This practice is increasingly relevant with the new Gmail and Yahoo email sender requirements.
When an ESP implements double DKIM signing, they ensure that regardless of which client domain is sending, the email also carries a valid DKIM signature from the ESP's infrastructure domain. This ESP domain can then be enrolled in Yahoo's CFL program. As a result, all complaints generated by emails sent through that ESP, across all their clients, will be reported back to the ESP's registered FBL address. This centralizes the collection of complaint data, making it far more manageable for large-scale operations.
The order in which the DKIM signatures appear in the email header generally does not affect their validity or how mailbox providers process them. Both signatures are typically checked independently. For example, an email might pass DKIM authentication for the client's domain and also for the ESP's domain. Yahoo's system simply looks for any valid DKIM signature from an enrolled domain to send back FBL data.
This approach provides a significant advantage for ESPs managing many clients, as it simplifies the process of receiving crucial complaint feedback. It ensures that even if a client hasn't registered their domain with Yahoo's FBL, the ESP still receives the necessary data to maintain a healthy sending environment for its shared IPs and domains. This method plays a key role in understanding overall platform performance and proactively addressing potential issues.
The primary benefit of using double DKIM signing for ESPs is the simplified and consolidated collection of Yahoo FBL data. Instead of requiring each client to enroll their domain in Yahoo's CFL, the ESP can manage a single enrollment for their own signing domain, effectively covering all mail sent through their platform. This drastically reduces administrative overhead and ensures comprehensive complaint visibility across their entire sending infrastructure.
Benefit
Explanation
Simplified FBL management
ESPs can register a single domain to receive complaint feedback for all emails sent through their platform, streamlining data collection.
Comprehensive complaint data
Ensures all client mail is monitored for complaints, regardless of individual client domain enrollment status.
Centralized reputation insights
Aggregated FBL data provides a holistic view of overall sending performance and potential issues across the ESP's user base.
Enhanced deliverability management
Quicker identification and resolution of spam complaints lead to better long-term inbox placement for all senders.
However, there are considerations. While double DKIM signing simplifies FBL collection, it introduces questions about which DKIM signature holds more weight in determining sender reputation. Mailbox providers like Google and Yahoo often consider multiple factors beyond just DKIM when evaluating sender reputation, including SPF authentication, DMARC policies, content quality, and user engagement. While the client's domain DKIM is typically the most impactful for their specific brand's reputation, the ESP's domain also contributes to the overall trust signals associated with the email.
Important note on reputation
The reputation associated with a double DKIM signed email is complex. While the client's domain DKIM signature often holds the primary influence over their specific sender reputation, the ESP's signing domain contributes to the overall perception of the sending infrastructure. A strong reputation for the ESP's domain can positively impact all emails sent through its platform, whereas a poor reputation can affect deliverability for many clients. ESPs should strive to maintain high standards for both their own domains and their clients' domains.
ESPs must carefully balance the convenience of centralized FBL data with the nuanced impact on individual client reputations. It's essential to analyze the complaint data received via the ESP's DKIM domain and attribute it back to specific clients to help them improve their sending practices. This granular analysis is key to preventing one client's poor performance from negatively affecting the deliverability of others on shared infrastructure.
Impact on sender reputation
The presence of two DKIM signatures means that both the client's domain and the ESP's domain contribute, in varying degrees, to the overall sender reputation of an email. When a complaint is generated, it's typically tied back to the domain enrolled in the FBL. If the ESP's domain is enrolled and receives the complaint, that data informs Yahoo's assessment of the ESP's sending reputation. Simultaneously, the client's domain reputation is influenced by their direct sending practices and how their mail is perceived by recipients, including complaints.
It's a common misconception that one DKIM signature entirely overrides the other. In reality, mailbox providers assess all available authentication signals. The client's DKIM signature is crucial for DMARC alignment and establishing the individual brand's trustworthiness. The ESP's DKIM signature provides an additional layer of authentication and allows the ESP to monitor global complaint rates and manage the reputation of their shared sending infrastructure. Effective management of both is key to strong email deliverability.
Ultimately, while double DKIM signing is an effective technical solution for ESPs to collect vital FBL data, the core principles of good sending remain paramount. This includes maintaining clean mailing lists, sending relevant content, and ensuring robust email authentication (SPF, DKIM, DMARC) for both the client's and the ESP's domains. Consistent monitoring of email metrics is crucial.
Views from the trenches
Best practices
Always register your primary sending domains with Yahoo's Complaint Feedback Loop to receive essential complaint data.
Implement double DKIM signing for emails sent through your platform to simplify FBL data collection across all clients.
Ensure both your ESP domain and client domains have valid DKIM records to maximize authentication success and deliverability.
Regularly monitor FBL data to identify trends, problematic campaigns, and list quality issues, allowing for proactive adjustments.
Educate clients on the importance of maintaining good sender practices, as their domain reputation impacts overall deliverability.
Common pitfalls
Failing to register domains with Yahoo's CFL, leading to a lack of visibility into recipient complaints and potential deliverability issues.
Overlooking the impact of individual client sending practices on the ESP's overall shared infrastructure reputation.
Not adequately processing ARF reports, which can result in missed opportunities to identify and address spam complaints.
Assuming that double DKIM signing completely negates the need for client-side reputation management, when both contribute.
Neglecting to monitor other reputation signals beyond FBLs, such as bounce rates and engagement metrics, which are also vital.
Expert tips
Establish clear guidelines for client onboarding regarding DKIM setup and FBL enrollment to ensure compliance and effective monitoring.
Leverage automated systems for processing FBL data to quickly identify and action on spam complaints, such as suppressing problematic subscribers.
Segment your sending IPs and domains based on client behavior and reputation, minimizing the impact of high-complaint senders on others.
Continuously analyze email authentication reports (SPF, DKIM, DMARC) alongside FBL data to gain a comprehensive understanding of deliverability challenges.
Consider implementing selective double DKIM signing for specific client cohorts or sending types if granular control is needed over FBL reporting.
Expert view
Expert from Email Geeks says there are two main approaches for collecting Yahoo complaints: double DKIM signing or independently registering each customer's domain for FBLs.
2021-11-24 - Email Geeks
Expert view
Expert from Email Geeks says when double DKIM signing is used, the ESP signature may not be as impactful on reputation as the more specific client-side signature. ESPs also have other reputation markers like message IDs and unsubscribe links.
2021-11-24 - Email Geeks
Conclusion
For ESPs, effectively collecting Yahoo FBL data is paramount for maintaining a healthy sending reputation and ensuring high inbox placement rates. Double DKIM signing offers a pragmatic and scalable solution, allowing ESPs to centralize complaint reporting while still enabling clients to establish their unique brand reputation. By understanding how Yahoo's domain-based FBL operates and implementing dual signing correctly, ESPs can gain invaluable insights into recipient feedback, proactively manage issues, and ultimately enhance the deliverability of all emails sent through their platform.
Google and 
Yahoo often consider multiple factors beyond just DKIM when evaluating sender reputation, including SPF authentication, DMARC policies, content quality, and user engagement. While the client's domain DKIM is typically the most impactful for their specific brand's reputation, the ESP's domain also contributes to the overall trust signals associated with the email.
