Gmail feedback loop (FBL) overview

Gmail offers a feedback loop (FBL) to high volume senders and email service providers (ESPs). The Gmail FBL is not like those offered by other mailbox providers, because it does not send complaints in Abuse Reporting Format (ARF). Gmail's FBL only provides a report with aggregated spam statistics for recipients based on specific identifiers in the header, in order to protect their users' privacy.

It is important to note that some lower volume senders may not see feedback loop data, even if all requirements for implementing the feedback loop are met. Gmail requires senders to hit an undisclosed volume and spam complaint threshold for data to be available. So, large volume senders and ESPs typically benefit the most from Gmail's feedback loop data.

Gmail only provides aggregate information
Gmail's Feedback Loop (FBL) notifications are only sent as aggregate complaint rates. They are based on emails that reach the inbox, as opposed to the total number of emails sent. Actual spam complaints made by a subscriber are not available.  

The Gmail FBL only sends aggregate data because it is designed to help high volume senders identify abuse that may be generated from their systems.  It is not intended to be used as a delivery troubleshooting or complaint removal tool for senders.

Benefit to senders
The Gmail FBL gives insight into your campaigns that might receive an extremely high number of spam reports from Gmail users. 

Senders using the Gmail FBL are required to:

  • Sign up with Gmail's Postmaster tools.
  • Authenticate with DomainKeys Identified Mail (DKIM)
  • Publish a Sender Policy Framework (SPF) record.
    • Ensure all sending IPs are published in the SPF record, have a valid reverse DNS (PTR) record and resolves to a valid hostname (the DKIM domain is preferred)
  • Adhere to a Domain-based Message Authentication, Reporting, and Conformance (DMARC) policy 

Gmail also encourages senders to comply with the Gmail Bulk Sender guidelines.

How to implement

To implement the feedback loop, Gmail requires that you insert a special header called the Feedback-ID.

Header format
The header is required in either of the following formats:

  • Feedback-ID: a:b:c:ESPid (for the ESP)
  • Feedback-ID: a:b:c:Senderid (for the high volume sender)


  • Feedback-ID is the name of the header to be embedded.
  • a, b, c are optional fields that can be used by the ESP or high volume sender to embed up to 3 identifiers of your choice, such as campaign, segment, and customer. These identifiers may change based on the attributes you wish to track.
  • ESPid or Senderid is a required, unique identifier chosen by the sender and can range from 5 to 15 characters. Use the same ESPid or Senderid identifier across all email streams sent to Gmail.

For example:

Feedback-ID: CampaignID:SegmentID:Senderid

Feedback-ID: Holiday1:NewCustomers:CompanyX

Implementation tips

  • Be sure not to mix identifiers across different campaigns (except the ESPid or Senderid, which should be the same across all email streams) or the data may be aggregated across unrelated identifiers.
  • Add your DKIM domain to the Gmail Postmaster Tools so you can access the data.

When you implement the headers, Gmail also sends a sender a comma-separated value (CSV) report that contains data in columns showing:

  • Date
  • Identifier
  • Complaint %
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