What ESPs use Validity's FBLs and what is the status of Comcast data and DKIM FBL implementation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 11 May 2025
Updated 19 Aug 2025
8 min read
When it comes to email deliverability, feedback loops (FBLs) are indispensable. They act as a critical communication channel between mailbox providers (MBPs) and email senders, allowing senders to identify and suppress users who mark their emails as spam. This feedback is vital for maintaining a healthy sender reputation and ensuring future emails reach the inbox. Without it, senders would be largely blind to user complaints, leading to lower inbox placement rates and potential blocklist (or blacklist) issues. This is why many Email Service Providers (ESPs) prioritize integrating with various FBL programs.
Validity, a prominent name in the email deliverability space, offers its own FBL services, particularly as part of its certification programs. The question often arises among email professionals about which specific ESPs leverage Validity's FBLs and how widely adopted they are. While direct, public lists of participating ESPs are generally not available due to commercial agreements and competitive reasons, it's understood that many larger ESPs with sophisticated deliverability teams do engage with Validity's services, especially if they are part of their Sender Certification program.
Beyond Validity, the landscape of FBLs is constantly evolving. A recurring point of interest and sometimes frustration for deliverability professionals is the status of specific mailbox providers, such as Comcast. There's also been considerable discussion surrounding the implementation of DKIM-based FBLs, a newer proposed mechanism designed to streamline feedback for DKIM signers. I often find myself looking for updates on these developments to understand their impact on email programs. I'll explore the current state of these FBL initiatives and what it means for senders.
For ESPs, joining FBLs is a standard practice for maintaining healthy email sending operations. These programs allow ESPs to receive automated reports when their subscribers click the This is spam button. This data is then used to automatically suppress (remove) those users from mailing lists, preventing further complaints and protecting sender reputation. Validity's FBLs, often associated with their Return Path certification, provide insights into complaint rates across various participating mailbox providers.
While Validity offers a significant FBL service, the actual number of ESPs that have formally signed up or are actively using their FBL data is not publicly disclosed. From what I've observed in the industry, participation often depends on an ESP's specific needs, their client base, and whether they are pursuing a broader certification. Some ESPs might find the data volume from certain FBLs (including some under Validity) to be a tiny trickle that doesn't significantly alter their suppression practices compared to other larger FBLs like Gmail's Feedback Loop or Yahoo's Feedback Loop.
It’s worth noting that participation in a paid program like Validity Sender Certification might provide access to FBL data from MBPs that do not offer public, free FBLs, making it a valuable addition for certain high-volume senders. However, for many ESPs, the cost-benefit analysis of such services means they opt to focus on widely available and free FBLs where the complaint volume is typically much higher.
The purpose of FBLs
Feedback loops are a cornerstone of email deliverability, providing senders with crucial data on spam complaints directly from mailbox providers. This allows for prompt removal of unengaged or complaining subscribers, preventing negative impacts on sender reputation. Without FBLs, senders would lack visibility into a critical aspect of recipient engagement, making it much harder to manage their sending practices effectively.
How ESPs utilize FBL data
ESPs typically automate the processing of FBL data to update suppression lists in real-time. This proactive approach helps to maintain good standing with ISPs and avoid being placed on various email blocklists or blacklists. Understanding which mailbox providers offer FBLs is essential for comprehensive deliverability management.
The status of Comcast data
Comcast (now Xfinity Mail) has long been a key player whose FBL data is highly coveted by deliverability professionals. Historically, obtaining complaint data from Comcast has been challenging, leading to many ESPs wishing for a direct and reliable feed. Their postmaster site has had an FBL registration page that mentions both IP-based and DKIM-based feedback loops.
However, the practical availability and utility of this data for ESPs have been inconsistent. Many email service providers (ESPs) report that even with registration, the complaint data received from Comcast (or the lack thereof) is minimal or non-existent. This makes it difficult to effectively manage suppression lists for Comcast subscribers based solely on their FBL, pushing ESPs to rely more on overall engagement metrics and other, more robust FBLs.
The desire for accurate Comcast data stems from their significant user base. Having reliable FBL data from major ISPs helps ESPs proactively identify and address spam complaints, preventing their sending IPs from being blocklisted (or blacklisted) and ensuring high inbox placement. Until a more consistent and accessible solution is widely adopted, Comcast data remains a missing piece of the puzzle for many in the deliverability community.
DKIM FBL implementation: the current state
The concept of DKIM-based FBLs (Feedback Loops) is an exciting development, aiming to provide feedback reports directly to the entity responsible for the DKIM signature, which is often the sending domain rather than just the sending IP address. This would be a significant step forward, offering more granular data that aligns with modern email authentication practices like SPF, DKIM, and DMARC.
The relevant draft, Email Feedback Reports for DKIM Signers, outlines a mechanism where discovery is made via DNS, using elements within the DKIM signature. The implementation of this draft is currently in its very early stages. While it seems like a straightforward DNS update from a technical perspective for signers, widespread adoption by mailbox providers (MBPs) is still far off.
As of now, the draft has received minimal feedback, and its specifications could still change substantially. This means that, for practical purposes, most ESPs and senders will not be receiving DKIM-based FBL reports anytime soon, as ISPs are unlikely to code up support for a standard that is not yet stable. Therefore, despite the potential benefits, the current adoption numbers for DKIM FBLs are effectively zero.
Traditional IP-based FBLs
Scope: Report complaints linked directly to the sending IP address.
Implementation: Requires senders to register specific IPs with each individual mailbox provider.
Data format: Often uses Abuse Reporting Format (ARF) emails.
Pros of IP-based FBLs
Established: Most major mailbox providers already offer them.
Direct: Provides clear complaint signals for the sending infrastructure.
Proposed DKIM-based FBLs
Scope: Report complaints linked to the DKIM signing domain.
Implementation: Relies on DNS records associated with the DKIM signature.
Data format: Also expected to use ARF, but discovery is different.
Pros of DKIM-based FBLs
Domain-centric: Better aligns with domain reputation and DMARC.
Simplified management: Could reduce complexity for senders with multiple IPs.
The broader FBL ecosystem and future outlook
The email deliverability ecosystem is always evolving, and FBLs are a core part of that. While Validity's FBLs serve a specific niche, particularly for certified senders, the broader market relies on a combination of proprietary and public FBLs. The discussions around new standards, like the DKIM-based FBL, highlight a collective effort to enhance the efficiency and accuracy of spam complaint reporting. I remain optimistic about future advancements that could provide more comprehensive data to senders.
Regardless of the specific FBL, the underlying goal remains consistent: to help senders maintain a positive reputation and avoid blocklists (or blacklists). By understanding where complaints originate, ESPs and senders can refine their list hygiene, content, and sending practices, ultimately improving their overall email deliverability. It's a continuous process of monitoring, adapting, and optimizing.
For ESPs, it's essential to stay informed about these changes and to ensure their infrastructure can adapt. This includes being ready to implement new DNS records or data processing mechanisms as standards evolve. While the landscape of FBLs can be complex, robust monitoring and quick adaptation are key to navigating it successfully and ensuring emails consistently reach the inbox.
FBL Provider
Type
Key Features
Data Coverage
Validity
Proprietary (part of certification)
Includes data from various ISPs, often more detailed reports
Specific to participating mailbox providers under Validity's umbrella
Google Postmaster Tools
Proprietary (IP/domain-based)
Provides aggregated complaint rates, reputation, DMARC data
Gmail traffic
Outlook.com (Junk Mail Reporting)
Proprietary (IP-based)
Offers detailed ARF reports for registered IPs
Outlook.com, Hotmail, Live traffic
Yahoo (formerly Oath)
Proprietary (IP/DKIM-based)
Provides FBL data, including a newer Complaint Feedback Loop (CFL)
Yahoo, AOL, and other Oath properties
Views from the trenches
Best practices
Actively participate in as many relevant FBLs as possible to gather comprehensive complaint data for your sending infrastructure.
Automate the processing of FBL reports to ensure immediate suppression of complaining users from your mailing lists.
Continuously monitor your sender reputation metrics, as FBL data is a key component in assessing your email program's health.
Common pitfalls
Failing to suppress users who file complaints, leading to continued sending to disengaged recipients and reputation damage.
Over-relying on a single FBL provider, potentially missing critical complaint data from other significant mailbox providers.
Expert tips
FBLs are essential for maintaining domain reputation, as high complaint rates significantly increase the risk of being placed on a blacklist (or blocklist).
DKIM-based FBLs are designed to offer more granular data, aligning with modern email authentication protocols.
Even if FBL data from a specific provider is a 'tiny trickle,' every piece of complaint feedback contributes to a clearer picture of your audience's engagement.
Expert view
Expert from Email Geeks says they didn't sign up for Validity's FBLs, and it hasn't changed anything for them. They mentioned they wished they could get Comcast data because other FBLs provided such a tiny trickle that it didn't matter.
2024-05-10 - Email Geeks
Expert view
Expert from Email Geeks says they are optimistic that the DKIM FBL draft (v2) will see some progress, as it was released recently.
2024-05-10 - Email Geeks
Navigating the FBL future
The landscape of FBLs is a dynamic and essential part of email deliverability. While specific ESP adoption numbers for Validity's FBLs remain private, their value lies in providing additional complaint data, particularly for senders focused on certification. The status of Comcast data continues to be a point of concern for ESPs, with many wishing for more comprehensive and reliable complaint feedback.
Moreover, the proposed DKIM FBL represents a promising future direction for complaint reporting, aligning more closely with modern authentication standards. However, its implementation is still in the draft stage, meaning practical adoption by major ISPs like Comcast is not yet a reality. As an industry, we continue to push for standards that will make email sending more transparent and reliable for everyone.
Understanding these nuances helps deliverability professionals strategize effectively. It underscores the importance of a multi-faceted approach to feedback loop management, leveraging all available FBLs, closely monitoring reputation metrics, and staying informed about emerging standards. This proactive stance ensures that email programs can adapt and thrive in an ever-changing environment, keeping messages out of the spam folder and in the inbox.
Xfinity Mail) has long been a key player whose FBL data is highly coveted by deliverability professionals. Historically, obtaining complaint data from Comcast has been challenging, leading to many ESPs wishing for a direct and reliable feed. Their postmaster site has had an FBL registration page that mentions both IP-based and DKIM-based feedback loops.
Validity
Google Postmaster Tools
Outlook.com (Junk Mail Reporting)
