What is an Email Feedback Loop (FBL)?

Understanding Feedback Loops

What is an email feedback loop?

An email feedback loop, also known as a complaint feedback loop, enables an Internet service provider (ISP) to inform a sending organization about spam complaints submitted by recipients of their messages. Doing so enables the sender to remove those email subscribers from their lists and ensure they won’t continue to receive unwanted messages and file more spam complaints.

Removing email subscribers who have filed complaints also helps keep a sending organization’s spam complaint rate low. The lower a sender’s spam complaint rate is, the better their chances of successfully reaching recipients’ inboxes in the future, because ISPs won’t be as likely to quarantine or reject their messages.

How does an email feedback loop work?

Each ISP offers its own feedback loop (FBL), and it’s incumbent on a sending organization, whether that’s an email service provider (ESP) like SparkPost or a business handling email on its own, to register for each one. A typical FBL uses a button or a link to a form that allows a recipient to register a complaint about unwanted email.

Some ISPs send the details of each individual spam complaint to the sending organization while others, such as Gmail, provide aggregate data that doesn’t say which recipients filed complaints. The business that generated the email and should immediately remove people who file spam complaints from their mailing list, preferably through an automated process that eliminates the potential for human error.

Why should you care about an FBL?

It’s critical that you maintain a suppression list, or exclusion list, and add people to it when they file spam complaints or submit unsubscription requests, or when their email addresses produce a hard bounce, which means the address is no longer valid. (A soft bounce means there was a temporary error or delay, so the address can be tried again.)

If you repeatedly send email to people who should be on your suppression list, your sending reputation will take a hit. ISPs will limit delivery of email from you to inboxes on their network, and will refuse all messages from you if your sending reputation drops too low.

How does SparkPost work with FBLs?

SparkPost has registered with all the major ISPs’ feedback loops. We support two types of suppression lists:

An account-specific list available via the Suppression List API (subaccounts have individual lists that are separate from the master account list)
A global list that spans all SparkPost customers (it is not accessible via the Suppression List API)

When a message is injected using either SMTP or HTTP, SparkPost checks the recipient’s email address against both lists. If it’s found on one of the lists, SparkPost automatically refuses to send the message to that person.

SparkPost adds email addresses to the account-specific list or both lists in the following circumstances:

An ISP sends a spam complaint or FBL: SparkPost automatically adds that email address to the account-specific list.
An ISP sends a hard bounce message: SparkPost automatically adds that email address to the account-specific list.
A recipient sends an unsubscribe request: SparkPost automatically includes an unsubscribe link in each message. Recipients can use it or the List-Unsubscribe header, which is not included in transactional messages. SparkPost automatically adds that email address to the account-specific list.
A recipient contacts SparkPost: SparkPost’s Compliance Team adds a recipient’s email address to the account-specific list if they contact us to request that you no longer send them emails.
You use the Suppression List API: SparkPost’s REST API lets you insert or update a single entry or multiple entries in your suppression list, as well as check the suppression status for specific recipients and remove email addresses from your list.
You use the Suppression List UI: In the user interface, you can choose your suppression list and manually manage it.

Learn More about FBLs

Dig deeper into useful FBL resources

Here are some resources that will help you dig deeper into FBLs:

Wikipedia’s entry on email FBLs: This is a good overview of how FBLs work.
A list of IP-based, aggregated, and domain-based FBLs: This list is maintained by the Messaging, Malware and Mobile Anti-Abuse Working Group.

Get help with FBLs on SparkPost

The SparkPost Support Center is a good place to start learning about SparkPost in general.

Using Suppression Lists: Learn how to use FBLs with SparkPost.
Suppression List: Information for developers.
554 5.7.1 recipient address was suppressed due to customer policy: An explanation of that error message.
Bounces: An explanation of how SparkPost displays bounces in the user interface.