Google can name the send.
Some days it publishes nothing.

One header field, printed on every send.

Klaviyo stamps a header called Feedback-ID on each message it sends. Google reads that header, groups the day's complaints by the identifiers inside it, and, when a group is large enough, publishes the identifier back in Postmaster Tools with its own spam ratio. That published string is the difference between knowing which send caused a spike and guessing at it.

The first three rows are Google's specification. The last three are one anonymized send from the example incident. Klaviyo decides what goes in the optional fields; the shape is Google's.

Klaviyo sets it. There is nothing here for you to configure.

This is the rare deliverability control that is already switched on. Klaviyo writes the header; Google decides whether anything comes back.

Why the panel is empty on some days.

Publication is volume-gated. Google sets the gate, and does not say where it sits.

• volumeBefore content matters, size does. An identifier has to appear in enough of that day's mail to be eligible at all, so a small send can produce an appalling complaint rate and still never surface here.
• reportsEligibility also needs enough distinct user spam reports on the same day. A handful of annoyed recipients is not an aggregate.
• googleGoogle's wording: “For a given day's traffic, FBL reports are generated only if a given Identifier is present in a certain volume of mails as well as in distinct user spam reports.”
• undisclosedNeither number is published. Google states that a threshold exists and never states where it sits, so no tool, this one included, can tell you how close a send came to clearing it. Anyone quoting you a figure is guessing.
• empty fieldGoogle also generates nothing when the required SenderId is blank: “If the SenderId is empty, no data will be generated.” Klaviyo fills that field on every send, so this particular failure mode is not yours to worry about.
• measuredOne account, for scale: on a Klaviyo sender running roughly ten million emails per ninety days, Google published identifiers on 63 of 86 days, and on every day the Gmail spam rate was elevated. That is one account's coverage, reported as an illustration and not as a promise. A smaller sender will see far less, and may see none.

Quoted from Google's Feedback Loop and Postmaster Tools dashboards documentation.

On the days Google publishes nothing, the investigation still runs.

The Identified tier goes away with the identifier. Three other evidence sets do not, and every finding is labelled by which of them carried it.

None of these earns the Identified label. Identified means Google named an identifier and that identifier contained a Klaviyo ID. Nothing substitutes for it, so a finding built on the three signals above is labelled Likely or Possible, and states which signal it rests on.

What you control, and what you do not.

Two of these are worth doing this week. The rest is worth knowing so you stop chasing it.

• You cannot raise Google's threshold. It is not a setting, not a support request and not something a paid tool unlocks. It is Google's aggregation rule applied to your traffic, and it applies the same way to everyone.
• You cannot backfill a missing day. Postmaster publishes what it published. If an identifier did not clear the gate on 5 April, it does not arrive later, and SpamSpike will not pretend otherwise.
• You cannot make Klaviyo see the Gmail complaint. Klaviyo's own wording: “Gmail does not provide spam complaint feedback directly to email service providers (ESPs) like Klaviyo.” A near-zero Klaviyo complaint number during a Gmail spike is not Klaviyo being wrong; it is Klaviyo never being told.
• You can keep sending volume steady. Erratic volume moves your identifiers in and out of eligibility, so coverage thins exactly when you want it most. Consistent daily volume gives Google more days on which it is able to publish anything.
• You can check that the tracked domain is the sending domain. Postmaster reports on the domain you verified. If Klaviyo sends from send.yourbrand.com and Postmaster tracks yourbrand.com, the Feedback Loop panel stays empty no matter how your sends perform. This is the single most common reason the panel looks broken.
• You can understand why per-message IDs are useless here. Klaviyo already handles this, but the rule explains the whole design: Google says a sender “should not use a parameter that will be unique across every single mail”. An identifier that never repeats can never aggregate, so it can never be published.