If you’re a large volume sender, you can use the FeedBack Loop (FBL) to identify campaigns in your traffic that are getting a high volume of complaints from Gmail users. The FBL is particularly useful to ESPs to detect abuse of their services.
Note: FBL data will only pertain to @gmail.com recipients.
How to implement the FBL
Feedback-ID: Name of the header to be embedded.
a, b, c: Optional fields that can be used by the sender to embed up to three identifiers (campaign/customer/other).
SenderId: Mandatory unique identifier (5-15 characters) chosen by the sender. It should be consistent across the mail stream.
About the data
The aggregate data will be generated for the first 4 fields (as separated by ‘:’) of the Feedback-ID: , starting from the right side. If the SenderId is empty, no data will be generated. If another field is empty, data will be generated for the rest of the fields.
In order to prevent spoofing of the Feedback-ID, the traffic being sent to Gmail needs to be DKIM signed by a domain owned (or controlled) by the sender, after the addition of this header. This domain should be added and verified to the Gmail Postmaster Tools, so that the sender can access the FBL data.
- Senders should ensure that their traffic has only one such verified header.
- Senders will have to publish the IPs from which they’re sending mail in the SPF records of their signing domains. The sending IPs must have PTR records and resolve to a valid hostname (preferably one of the DKIM domains).
- 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.
- FBL data will be aggregated on each identifier independently. This also allows for the use of less than 3 Identifiers, if needed.
- For a given day's traffic, the sender should ensure that the identifiers across fields not repeated, so that data is not aggregated across unrelated Identifiers. If there is a concern about the uniqueness of the identifier namespace, or if the sender prefers for the data to be grouped between two Identifiers, the hash of one identifier can be appended to the other.
- When choosing the identifiers, the sender should not use a parameter that will be unique across every single mail (for example, a unique Message-ID).
Below is an example for illustration:
CampaignIDX: Campaign identifier specific to Customer2 and is unique across the board (i.e., no two customers share the same campaign ID).
CustomerID2: Unique customer identifier.
MailTypeID3: Identifier for the type of email (e.g. a newsletter vs a product update) and can be either unique or common across customers, based on how the sender wants to view the data.
SenderId: Sender's unique identifier and can be used for overall statistics.
In the above case, we would send the spam percentages for each of the four identifiers independently, if they had an unusual spam rate.