This article is part of a series that answers newsletter questions we often hear from users. Send us any questions you'd like us to answer!
Tapping an unsubscribe link in a newsletter is for amicable divorces. It is for emails that you once loved but no longer do, or lists that you joined without thinking it through. If you’re not mad at the sender, but need some space, unsubscribe. For hostile and antagonistic senders, whose emails egregiously violate the original pact, lawyer up with the Spam button.
Several things happen behind the scenes when you flag an email this way:
The spam filter tailored to your account (and likely your inbox provider’s platform-wide filter) is updated based on the characteristics of the flagged email.
A spam complaint is automatically generated by your inbox provider and filed with the sender’s email service provider (ESP).
After receiving a spam complaint, ESPs will immediately block the sender from emailing the address that filed the complaint.
If the individual sender’s complaint rate passes a certain threshold, their ESP may issue a warning, temporarily suspend their sending privileges, or permanently ban their account.
In extreme cases that involve the sender using an ESP’s shared domain to deliver emails, internet service providers and third-party spam organizations might flag or even block the ESP’s domain or IP address, impacting all the other users who send from it.
I’d argue that understanding the specifics of how complaint feedback loops work won’t help you write emails that are less likely to receive a complaint. But the knowledge is a reminder of how unique and awesome email is in a time when most internet-based communication is siloed and centralized.
Not one but two spam filters for you
The humble Spam button is an excellent vehicle for understanding the differences between inbox providers and email clients. For most people, Gmail is an inbox provider, meaning it supplies the backend infrastructure that actually sends and receives email. That’s different from email clients like Apple Mail or Thunderbird, which can only display messages that were first routed by an inbox provider.
Apps that can be either inbox provider or email client make the distinction fuzzy, though. You can, for example, sign up for a free Outlook.com email address to send and receive messages, or use the Outlook apps to connect to a Gmail address. So what happens when you click the Spam button in an email client connected to a third-party inbox provider? In most cases, you’ll be training two separate spam filters.
When you connect a Gmail address to Apple Mail, hitting Move to Junk does two things. First, it tells Gmail to move the flagged message into the Spam folder. And any message moved into Gmail’s Spam folder trains that account’s filter to screen emails with similar traits. Second, Apple Mail also uses Move to Junk as a signal for its own, on-device spam detection. Michael Ko over at Suped distinguishes overlapping filters by explaining that “Client-side settings primarily affect the local display and filtering, not the underlying server-level decisions made by iCloud Mail or other services like Gmail or Outlook.”
When you sign up for a brand new account with an inbox provider, emails might start showing up in Spam before you mark a single message as such. That is the platform-level filter at work. And while I couldn’t find explicit evidence that signals from one account influence another account’s spam filter, it seems exceptionally likely that some cross-pollination occurs, even if the weights heavily favor localized signals.
So, when you click Report Spam in Gmail, you’re fine-tuning an account-level detection algorithm and probably the inbox provider’s broader algorithm as well. Click that same button in an email client and you’ll add a third, app-level filter into the mix. All are bad for the affected sender’s deliverability, but far less of an issue than an official complaint.
Complaint feedback loops and the ones that get away
Despite a handful of attempts to standardize email abuse reports in the mid-2000s, there wasn’t official standardization until RFC 5965. “Mail operators are increasingly exchanging abuse reports among themselves and other parties. However, different operators have defined their own formats, and thus the receivers of these reports are forced to write custom software to interpret each of them.” So the IETF proposed a complaint system that standardized both human and machine readable reports.
Early complaint routing often relied on informal headers like `X-Complaints-To`, which is still recognized by some ESPs today. But most feedback loops are now handled by registering a sending domain and IP with various inbox providers in order to receive complaints. Either way, when you click the Report Spam button, your inbox provider will send complaint details, usually anonymized and in the Abuse Reporting Format, to the address specified either in the complaint headers or on file with the inbox provider.
Somewhat annoyingly, the timing and contents of a CFBL complaint vary based on which inbox provider is sending it. Take Gmail’s approach for instance, which aggregates and anonymizes complaints in Postmaster Tools rather than sending traditional per-recipient feedback loop reports. Yahoo (and its still-kicking AOL addresses) sends a detailed summary for each complaint. Microsoft’s Junk Mail Reporting Program for Outlook and Hotmail forwards the entire original email, with complaint details appended. And there are dozens of other inbox providers with their own slightly different approaches.
However, email clients are here to complicate things once again. Manually moving an email into Gmail’s Spam folder does not trigger a CFBL complaint, you have to click Report Spam to send one. “If a user reads their email in an IMAP client, like Apple Mail or Outlook, and drags the email to their junk folder, this action often does not trigger a feedback loop complaint,” Michael Ko confirms. Your only option in that case would be to log into your inbox provider and click the button there.
Clicking Junk in Thunderbird, or Move to Junk in Apple Mail, doesn’t accomplish much beyond training your legion of spam filters. With the Report… buttons in Gmail and Outlook.com, however, you can create a paper trail of your complaint. And the sender’s ESP is all but forced to do something about it.
The consequences of complaints
No single entity has complete control over email. And so, inbox and email service providers have coalesced around similar but not quite identical responses to spam complaints. Every reputable sending platform will automatically remove your address from a list you have complained about. In Buttondown, you can filter your subscribers by the Complained status in case that helps shed light on what went wrong.
Beyond suppression, most ESPs will put a sender on probation or suspend their account outright when its spam complaints reach a certain threshold. Every platform sets its own tolerances, which might be calculated by complaints against individual campaigns or over an account’s lifetime. The general average is that 1 complaint for every 1,000 emails (0.1%) puts a sender in the Deliverability Danger Zone and 3 out of every 1,000 emails (0.3%) is enough to warrant account suspension.
Keep in mind that newsletters with small lists are often exempt from these thresholds. A single complaint made against a sender with only 100 people on their list would be a whopping 10x past the Danger Zone! In a 2023 Google blog article about deliverability standards, a Gmail Product Manager wrote that “We’re introducing new requirements for bulk senders — those who send more than 5,000 messages to Gmail addresses in one day.” At Buttondown, we’ll reach out to a small sender who’s gotten a couple of complaints, but a one-off is probably just a recipient who wants to unsubscribe aggressively.
Finally, the most extreme and rare thing that can happen when you click the Spam button is that your complaint joins a pile that was one short of justifying a domain-level punishment for an ESP. Imagine that a newsletter with an exceptionally large list sent a particularly heinous email, and that newsletter arrived in recipients’ inboxes from an @buttondown.com address. Complaints would flood in. Gmail, Outlook, and other providers would rightfully question our content checkpoints and safeguards. It would be totally justified to throttle or temporarily block emails from buttondown.com until the issue was resolved.
Over at r/emaildeliverability, the user sendatscale warns that “ESPs with a lot of poor customers will have a poor reputation themselves, and you - even if you are a good sender - will be negatively affected.” A sender can mostly avoid the shared domain reputation problem by sending from a personal domain. Still, it’s worth pausing before flagging something as spam that is instead simply annoying or in bad taste. Because if it arrived from a platform’s domain, what seems like a way to angry-unsubscribe could impact indie, smalltime senders.
Spam is email history
If it sounds self-serving to recommend an unsubscribe link over a Spam button, consider that Spamhaus, one of the oldest and most respected campaigners against junk email, defines it as anything “the Recipient has not granted verifiable permission for the message to be sent.”
It’s fairly rare for spam to make it as far as anyone’s inbox nowadays. And newsletters are especially infrequent offenders thanks to double opt-in, CFBL avenues, and the alphabet soup of authentication policies. But the Spam button persists. And as much as we hope people don’t mistake it for feedback on merely uninteresting content, there’s nothing we can do to stop you. No one owns email and that’s exactly what makes it so wonderful.