How do FBL reports work with forwarding and IP addresses?
Michael Ko
Co-founder & CEO, Suped
Published 12 Jul 2025
Updated 12 Oct 2025
6 min read
Email deliverability is a complex landscape, and one of the crucial components for maintaining a good sender reputation is understanding Feedback Loops (FBLs). FBLs are mechanisms that Internet Service Providers (ISPs) use to inform email senders when their recipients mark an email as spam or junk. This feedback is invaluable, allowing senders to identify problematic campaigns, clean their mailing lists, and improve their sending practices.
However, the intricacies of FBL reporting can become particularly challenging when email forwarding is involved. When an email is forwarded, the path it takes to its final destination changes, and with it, the potential for FBL reports to be misattributed. This can create a significant headache for those who manage forwarding services or rely on them for their email infrastructure.
The core question often revolves around whether FBL reports are sent based on the original Return-Path address or the last hop IP address that delivered the email to the reporting ISP. This distinction has profound implications for how you monitor and react to spam complaints, especially when your emails are being forwarded through intermediate servers. We will explore these complexities to help you maintain optimal email deliverability, even with forwarding in the mix.
Understanding FBL mechanics and IP addresses
Feedback Loops are designed to provide timely data on spam complaints. When a recipient at an ISP, such as Google or Yahoo, clicks the "report spam" button, the ISP generates a complaint report. This report is then sent back to the address that delivered the email to their system. Typically, this is the email address registered with the FBL service, often tied to a specific IP address or domain.
Most FBL systems are primarily IP-based. This means that the complaint report is associated with the IP address of the server that made the final delivery to the mailbox provider. This makes sense from an ISP's perspective, as it allows them to identify and potentially block IP addresses that are consistently sending unwanted mail. Senders register their sending IPs with FBL providers to receive these vital reports, which inform them about their sender reputation.
The information contained in an FBL report often includes details that can help identify the offending email, such as the original message headers and sometimes the entire email content. This allows senders to pinpoint exactly which campaign or subscriber triggered the complaint. Understanding how FBLs function for Google and Oath is essential for effective management.
The forwarding paradox
The situation becomes considerably more complicated when emails are forwarded. When an email is sent to an address that then forwards it to another mailbox, the forwarding server acts as an intermediary. From the perspective of the final destination ISP, the forwarding server is the one that delivered the email, not the original sender's server. This creates a disconnect between the origin of the email and where the FBL complaint is directed.
Many forwarding services use Sender Rewriting Scheme (SRS) to address issues with SPF authentication during forwarding. While SRS helps ensure that forwarded emails still pass SPF checks, it doesn't necessarily alter the behavior of FBLs. The FBL report still typically goes to the IP address that made the final connection, which is the forwarding server's IP. This means a forwarding service might receive FBL reports for spam that originated from a completely different sender, simply because they were the last hop.
Direct Sending
Direct path: Sender's server directly delivers to recipient's ISP.
FBL attribution: Complaint reports are correctly sent to the original sender's registered IP/domain.
Reputation impact: Directly reflects the original sender's sending practices.
With Forwarding
Indirect path: Sender's server delivers to forwarding server, which then delivers to recipient's ISP.
FBL attribution: Complaint reports are sent to the forwarding server's registered IP, even if they didn't originate the spam.
Reputation impact: Forwarding server's reputation can suffer due to others' spam, leading to blocklisting (or blocklisting).
This forwarding problem can severely impact the deliverability of the forwarding server, as it accumulates spam complaints for mail it didn't originate. It's a delicate balance for providers whose entire service hinges on forwarding emails, as they must ensure legitimate mail reaches its destination without inadvertently absorbing the reputation hit from spam. The IETF draft on FBL address headers is an attempt to mitigate this by allowing senders to specify a complaint feedback loop address.
Strategies for managing forwarded FBLs
To navigate the complexities of FBLs and forwarding, a multi-faceted approach is often required. For those operating forwarding services, one key strategy is to educate users on how to handle unwanted emails. Encouraging them to unsubscribe or filter rather than marking forwarded emails as spam can help protect the forwarding server's reputation.
Proactive steps for forwarders
User education: Instruct users to avoid reporting forwarded emails as spam to prevent misattributed complaints.
Source identification: Implement systems to trace the original sender when FBLs are received, aiding in spam source identification.
FBL registration: Ensure your forwarding IPs are registered for all relevant FBLs to receive complaint data, even if it's misattributed.
From a technical standpoint, robust email authentication protocols such as DMARC, SPF, and DKIM play a critical role. While forwarding can complicate their validation, especially for SPF, these protocols are essential for proving the legitimacy of an email. Understanding how email forwarding affects SPF, DKIM, and DMARC validation is crucial for managing your sender reputation effectively.
Ultimately, managing FBL reports in a forwarding scenario demands vigilance. Continuously monitoring your sending IPs and reviewing FBL data can help you quickly identify and address issues, even if the complaints are indirectly attributed to your infrastructure. Suped provides comprehensive DMARC monitoring and reporting, offering detailed insights into your email authentication status and helping you track email deliverability trends.
Views from the trenches
Best practices
Actively inform your users about the correct way to handle unwanted emails, such as unsubscribing or using filters, rather than marking them as spam.
Implement robust internal logging that captures the original sender's IP and Return-Path for forwarded emails, allowing you to trace the source of FBL complaints.
Ensure all sending IPs associated with your forwarding service are properly registered with relevant FBL programs to receive complaint data, even if it's indirect.
Common pitfalls
Ignoring FBL reports received by your forwarding IPs, which can lead to escalating blocklistings and impact all forwarded mail.
Failing to educate users about proper spam reporting, causing innocent forwarding servers to be penalized for third-party spam.
Not having a system in place to correlate FBL reports with the original sender, making it difficult to address the root cause of complaints.
Expert tips
Consider setting up a dedicated FBL processing system that automatically analyzes reports and flags high-volume senders, allowing for quick action.
If your service primarily forwards email, work with mailbox providers to whitelist your forwarding IPs or establish specific agreements to mitigate FBL impact.
Leverage DMARC aggregate reports alongside FBL data to gain a holistic view of your domain's authentication and potential abuse.
Expert view
Expert from Email Geeks says that spam reports based on IP addresses typically go to the address that delivered the email to the reporting system, which is standard for any FBL service.
2023-01-17 - Email Geeks
Expert view
Expert from Email Geeks notes that some email services stopped forwarding mail because receiving spam reports on their IPs led to significant blocklisting issues.
2023-01-17 - Email Geeks
Navigating FBLs in a complex email ecosystem
The interaction between FBL reports, email forwarding, and IP addresses presents a unique set of challenges for anyone involved in email delivery. While FBLs are vital tools for maintaining a healthy sender reputation, the way they attribute spam complaints can be problematic for forwarding services.
By understanding the underlying mechanisms of FBLs and implementing proactive strategies, such as user education and robust authentication measures, senders and forwarders can mitigate the risks. Leveraging tools like Suped's DMARC monitoring can provide the visibility needed to track these complex interactions and ensure your emails reach the inbox as intended.
Staying informed about the nuances of email deliverability and continuously adapting your practices are key to success. This includes diligently monitoring your FBLs, especially if you rely on email forwarding, to protect your sender reputation and ensure consistent email performance.
