Feedback-ID
A header Gmail reads to report spam complaint rates per campaign or customer in Postmaster Tools: up to three identifiers and a sender ID, separated by colons.
What it means
The format is Feedback-ID: a:b:c:SenderId. SenderId is mandatory, 5 to 15 characters, and should stay the same across the sender's mail stream; a, b and c are optional identifiers such as campaign, customer and message type. Gmail aggregates complaints for each identifier separately and reports those with an unusual spam rate in the Postmaster Tools Feedback Loop dashboard.
Google sets conditions. The header must be covered by a DKIM signature from a domain the sender owns and has verified in Postmaster Tools; the sending IPs must be in that domain's SPF record and have PTR records; only one such header should be present; and identifiers must not be unique per message. Data appears only when an identifier reaches enough volume and complaints, and covers only @gmail.com recipients.
Example
Feedback-ID: spring-sale:cust042:newsletter:examplesendRelated terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Google, Gmail Help: Feedback Loopsupport.google.com
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