What do "Identifiers Flagged" mean in Google Postmaster Tools Feedback Loop?
Michael Ko
Co-founder & CEO, Suped
Published 15 Jun 2025
Updated 19 Aug 2025
7 min read
When you're deeply involved in email deliverability, understanding the nuances of tools like Google Postmaster Tools is crucial. It provides invaluable insights into your email performance with Google's Gmail recipients. One specific area that often raises questions is the Feedback Loop section, particularly the metric labeled Identifiers Flagged.
This metric isn't just a number, it's a window into the specific segments of your email sending that are generating spam complaints. Understanding what this means and how to interpret it is vital for maintaining a healthy sender reputation and ensuring your messages reach the inbox, rather than landing in the spam folder or on a blocklist (or blacklist).
I've personally found that diving into these details helps pinpoint issues much faster than general spam rate graphs alone. It allows you to move from general observation to targeted action, which is key for effective email deliverability management.
What are feedback loop identifiers?
The Feedback Loop (FBL) system is a critical component for legitimate bulk senders. When a recipient marks an email as spam, Internet Service Providers (ISPs) like Gmail aggregate these complaints and provide a mechanism for senders to receive reports. This allows senders to identify problematic campaigns or mailing practices and take corrective action, ideally by removing the complaining subscriber from future mailings.
In the context of Google Postmaster Tools, Identifiers Flagged refers to the unique values within your Feedback-ID header that Google detects as being associated with spam complaints on a particular day. This header, which you implement in your outgoing emails, allows you to embed internal identifiers that help trace the source of a specific email. You can learn more about how Gmail's Feedback-ID helps analyze complaints for deeper insights.
For example, you might include identifiers for the client, campaign, and email batch (e.g., Feedback-ID: accountID:CampaignID:SlotID). When a user flags an email as spam, Gmail looks at this header. If enough complaints are attributed to a specific combination of these identifiers (or a single identifier if that's all you're using), that identifier will be flagged in your Postmaster Tools dashboard.
How Google identifies identifiers for reporting
Google's approach to identifying Identifiers Flagged is smart because it’s not limited to just the Feedback-ID header you provide. While that’s the primary source and best practice, Gmail can also identify any consistent element within your emails that consistently receives spam complaints. This could be anything from a specific URL or unique tracking parameter to a particular client ID or campaign name embedded in the email body or other headers.
The reasoning behind this flexibility is to ensure that senders, even those not fully leveraging the Feedback-ID header, still receive actionable data. This means if you have a recurring identifier, such as a client ID that’s always present, Google can pick up on it and associate complaints with it, even if it's not explicitly defined in your FBL header configuration. You can review the scope of Postmaster Tools Feedback Loop identifier spam rates for more context.
Example Feedback-ID Headerhttp
Feedback-ID: client123:campaignABC:batch789
It’s important to note that Gmail will only display identifiers if the volume of complaints associated with them is significant enough that they believe you won't be able to reverse-engineer individual user complaints from the data. This privacy-preserving measure ensures that while you get aggregate insights, you can't identify the specific users who complained.
Interpreting your "identifiers flagged" data
The count displayed under Identifiers Flagged in Google Postmaster Tools tells you the number of unique Feedback-ID values (or other identified consistent elements) that were associated with spam complaints on a particular day. A high number of flagged identifiers could indicate that spam complaints are widespread across many of your campaigns or sending segments.
It’s important to differentiate this from your overall spam rate. The spam rate tells you the percentage of your emails that recipients marked as spam, while the Identifiers Flagged metric tells you which specific segments of your sending are contributing to those complaints. Both metrics are crucial for a complete picture. For a more detailed look into this, consider what the identifier count in Google Postmaster Tools means.
Understanding the value
The raw number of Identifiers Flagged alone isn't always the most informative metric. What truly matters is diving into which identifiers are appearing and their associated spam rates. By clicking on a data point in the graph within Postmaster Tools, you can view a table that breaks down the specific identifiers flagged and their corresponding spam rates. This allows for granular troubleshooting and understanding how to interpret and identify spam complaints.
For instance, if Campaign A has a high spam rate associated with its identifier, while Campaign B has none, you know where to focus your efforts. This allows for precise adjustments to your email strategy, whether it's list segmentation, content changes, or sending frequency for that specific campaign. The team at ISIPP provides a good overview of viewing identifiers.
Understanding what is being flagged (whether it's an accountID, campaignID, or SlotID) is key. For example, if a specific SlotID (representing an email batch) consistently appears with high spam rates, it might indicate an issue with the list segment used for that batch or the content of those specific emails.
Actionable steps and common issues
Once you've identified which specific segments or identifiers are generating complaints, you can take targeted action. This could involve segmenting your audience more effectively, refining your content strategy for certain campaigns, or adjusting sending frequency. It's about moving beyond general deliverability fixes to address the root cause of the complaints.
Common reasons for flagged identifiers include poor list hygiene (sending to old or unengaged addresses), irrelevant or misleading content, sending too frequently, or lack of proper consent. These factors can quickly lead to recipients marking your emails as spam, which then gets reported back via the FBL system, potentially adding your domain to a blacklist (or blocklist).
Scenario 1: High volume, few identifiers
Issue: A broad complaint source, suggesting a general problem across most of your email stream. This indicates a widespread issue, not isolated to specific segments. Your overall sender reputation might be at risk.
Action: Investigate your entire list quality and overall content strategy. Re-evaluate your opt-in processes and consider a re-engagement campaign.
Scenario 2: Low volume, many identifiers
Issue: Many niche problems across various small segments. This means specific campaigns or client groups are underperforming, even if the overall spam rate isn't alarming.
Action: Conduct a granular content and audience review for each flagged identifier. Tailor messages and segment lists more precisely.
Solution 1: Improve list hygiene
Regularly: Clean inactive or unengaged users from your lists. This reduces the likelihood of spam complaints.
Implement: Double opt-in processes to ensure genuine interest and prevent spam traps.
Solution 2: Content and frequency
Tailor: Your email content to match subscriber expectations and interests. Avoid overly promotional or misleading subject lines.
Optimize: Your sending frequency. Too many emails can lead to fatigue and increased spam complaints, even if the content is relevant.
If you notice that your Google Postmaster Tools data for the Feedback Loop isn't populating, or if it shows a flat or zero graph, it's worth investigating. This could be due to low sending volume, an incorrect Feedback-ID implementation, or other technical issues. I recommend checking why your Gmail Postmaster Tools Spam Feedback Loop isn't populating to troubleshoot further.
Views from the trenches
Best practices
Ensure you consistently use a Feedback-ID header in all bulk email sends to Gmail.
Segment your Feedback-ID to include campaign or client identifiers for granular insights.
Monitor daily FBL data to quickly identify and address new spam complaint spikes.
Common pitfalls
Not implementing a Feedback-ID header, missing out on crucial complaint data.
Using overly broad Feedback-IDs that don't allow for specific problem identification.
Google may flag any consistent element in your email, not just the Feedback-ID, if it identifies a pattern related to spam complaints.
A spike in flagged identifiers often correlates with recent changes in content, list acquisition, or sending volume.
Don't solely focus on the 'Identifiers Flagged' count; always cross-reference it with the associated spam rate for each identifier.
Expert view
Expert from Email Geeks says that the identifiers flagged in Google Postmaster Tools should come from the feedback header, and I have typically seen it match in the past. It's worth checking if you are using more than one bulk sending platform, as this could cause discrepancies.
2018-11-22 - Email Geeks
Expert view
Expert from Email Geeks says that Google only shows an identifier for a header if the volume is great enough that they do not believe you would be able to reverse-engineer it, which is a privacy measure.
2018-11-22 - Email Geeks
Driving better inbox placement
The Identifiers Flagged metric in Google Postmaster Tools Feedback Loop is a powerful indicator of your email program's health. It goes beyond simple complaint rates, offering granular insights into which specific campaigns or segments are causing recipients to hit the spam button. By effectively utilizing this data, you can proactively identify and resolve issues, preventing widespread deliverability problems and safeguarding your sender reputation.
Regularly checking this dashboard, understanding the identified values, and taking swift action based on the reported spam rates are essential steps for any sender aiming for optimal inbox placement. This proactive approach ensures your emails reach their intended audience, fostering better engagement and overall success for your email campaigns.
Google's approach to identifying Identifiers Flagged is smart because it’s not limited to just the Feedback-ID header you provide. While that’s the primary source and best practice, Gmail can also identify any consistent element within your emails that consistently receives spam complaints. This could be anything from a specific URL or unique tracking parameter to a particular client ID or campaign name embedded in the email body or other headers.
