What do ESPs do when a subscriber marks an email as spam?
Matthew Whittaker
Co-founder & CTO, Suped
Published 8 Jul 2025
Updated 16 Aug 2025
6 min read
When a subscriber flags an email as spam, it sends a powerful signal that goes beyond a simple unsubscribe. Email Service Providers (ESPs) are on the front lines, processing these critical signals to protect their sending infrastructure and ensure deliverability for all their clients. This action triggers a series of responses within the ESP's system, designed to mitigate harm to sender reputation and prevent future unwanted mail.
Immediate actions and data management
The primary mechanism for ESPs to receive spam complaints is through Feedback Loops (FBLs). These are agreements established between major Mailbox Providers (MBPs) and ESPs. When a recipient clicks the "mark as spam" button in their email client, the MBP generates an FBL report and sends it back to the ESP responsible for the original email.
Most major MBPs, such as Yahoo and Microsoft (Outlook.com), participate in FBL programs. However, Gmail notably does not provide traditional FBLs, relying instead on data visible in their Postmaster Tools to indicate spam rates and other metrics. This distinction is crucial for ESPs and senders to understand when monitoring their complaint rates. You can learn more about this by checking which mailbox providers report spam complaints.
Once an ESP receives an FBL report, it processes the complaint. The goal is to immediately cease sending further emails to that specific recipient from the complaining sender. This rapid response is vital to protect the sender's reputation and prevent further negative signals from accumulating against their IP addresses and sending domains.
Immediate actions and data management
Upon receiving a spam complaint, ESPs typically take one of two main approaches to ensure no further mail is sent to the complaining recipient.
Suppression list approach
Many ESPs add the complaining email address to a suppression list. This list, often unique to each client or global across the ESP, permanently prevents further mailings to that recipient, regardless of future subscription attempts. This is a strong, definitive action to protect sender reputation.
While effective for compliance and deliverability, this can lead to issues if a user mistakenly reports an email as spam but later wishes to re-subscribe. Getting off a suppression list can be challenging, often requiring direct intervention from the ESP or brand support, which can be a source of frustration for the user and the brand's support team. For more details, consider why emails are suppressed after spam complaints.
Functional unsubscribe
Some ESPs might treat a spam complaint as a functional unsubscribe. In this scenario, the user is removed from the active mailing list, similar to a regular unsubscribe request. The ESP may or may not distinguish this from a standard unsubscribe in its internal records. This approach might offer more flexibility for re-subscription, depending on the ESP's specific policies and the brand's data management.
While seemingly less stringent, it's critical that the ESP still logs the complaint to inform the sender's reputation. A legitimate ESP will never do nothing when a subscriber marks an email as spam, as ignoring these signals would rapidly lead to severe deliverability issues for all clients.
Regardless of the specific method, the key objective for the ESP is to prevent further email from being sent to that recipient. This protects both the sender from accumulating more complaints and the overall shared IP reputation of the ESP.
The ripple effect on sender reputation
Spam complaints directly impact a sender's reputation, affecting both their IP address and domain. Mailbox providers use complex algorithms that factor in complaint rates, among other metrics, to determine whether future emails will reach the inbox, land in the spam folder, or be blocked entirely. A high spam complaint rate signals to MBPs that the sender might be sending unwanted mail, leading to more aggressive filtering.
Many industry benchmarks suggest that a spam complaint rate exceeding 0.1% to 0.3% can be problematic. For example, Google's Postmaster Tools provide general spam rate thresholds. Exceeding these thresholds can severely damage your sender reputation, making it much harder to reach the inbox. ESPs actively monitor these rates, as their overall infrastructure reputation is at stake. For more on this, read about what is considered a high spam rate.
Persistent high complaint rates can lead to a sender's IP address or domain being added to a blocklist (or blacklist), preventing emails from being delivered at all. ESPs may even suspend or terminate accounts that consistently generate excessive complaints, to protect the deliverability of their other clients. Understanding and managing your spam complaint rate is paramount for maintaining good email deliverability. This also highlights how spam complaints impact email deliverability.
Navigating re-engagement and visibility
The dynamic between ESPs, MBPs, and senders regarding spam complaints can be intricate, often creating challenges for all parties involved. A common frustration arises when a recipient, who might have mistakenly marked an email as spam, later tries to re-subscribe. If the ESP has placed that address on a suppression list, the brand's customer support (Tier 1) may not have direct visibility into this, leading to a confusing and frustrating experience for the customer who believes they are subscribed but aren't receiving emails.
ESPs are diligent in making complaint data available to their clients, usually through their platform interfaces or reports. However, the onus is on the brand to effectively integrate and act on this data. Brands that simply re-upload their entire mailing list without reconciling it against ESP-provided suppression data risk repeatedly mailing to recipients who have complained, which further damages their sender reputation.
Effective management of spam complaints requires a proactive approach, including regular list hygiene, clear and prominent unsubscribe options, and careful monitoring of deliverability metrics. Implementing robust DMARC monitoring can also provide valuable insights into your email stream's authentication and potential abuse. Brands must prioritize data synchronization with their ESPs to avoid unintended re-mailings and maintain a healthy sender reputation. For more on this, learn how to manage email spam complaints.
Views from the trenches
Best practices
Actively use feedback loops to identify and remove complaining subscribers from your mailing lists.
Maintain a robust, per-brand suppression list to prevent accidental re-subscriptions and repeat complaints.
Ensure clear, single-click unsubscribe options are readily available in all emails to reduce spam reports from disengaged subscribers.
Regularly clean your email lists by removing inactive subscribers and bounced addresses to improve overall deliverability and reduce complaint rates.
Educate your customer support team on how ESP suppression lists work and how to handle inquiries from customers who report not receiving emails after a complaint.
Common pitfalls
Ignoring FBL reports or failing to implement automated processes for handling spam complaints, leading to continued mailing to unhappy recipients.
Re-uploading email lists without first removing addresses that have previously complained, causing a cycle of re-complaints and reputation damage.
Lacking a clear distinction between a general unsubscribe and a spam complaint within your internal database, which can obscure critical deliverability insights.
Not providing your customer support team with access to ESP data or sufficient training to resolve issues related to email suppression due to complaints.
Assuming that all mailbox providers (MBPs) provide detailed FBL data; some, like Gmail, rely on other metrics and Postmaster Tools.
Expert tips
Monitor your sender reputation metrics, including spam complaint rates, through ESP dashboards and MBP Postmaster Tools regularly.
Segment your audience and tailor content to ensure relevance, reducing the likelihood of recipients marking your emails as spam.
Implement email authentication protocols like SPF, DKIM, and DMARC to build trust with MBPs and protect your sending domain from spoofing.
Periodically review your email acquisition methods to ensure that subscribers are genuinely opting in and have clear expectations about the content they will receive.
Consider transactional emails separately from marketing emails, ensuring that critical communications can still reach users even if they have opted out of marketing content.
Expert view
Expert from Email Geeks says ESPs prevent mailing to users who mark emails as spam, but the specific method varies. Some use special flags, while others simply unsubscribe the user, and no legitimate ESP would do nothing.
2021-10-21 - Email Geeks
Expert view
Expert from Email Geeks says there are ESPs that only unsubscribe rather than suppressing mail from that customer to that recipient, often due to infrastructure limitations, though suppression lists can be as simple as a database field.
2021-10-21 - Email Geeks
Maintaining a healthy sender reputation
The actions ESPs take when a subscriber marks an email as spam are fundamental to email deliverability. Whether through direct suppression or a functional unsubscribe, their primary objective is to prevent future unwanted mail and protect the sender's reputation.
For senders, understanding these processes is crucial. Proactive list management, ensuring clear unsubscribe paths, and diligently monitoring complaint rates are not just best practices, but necessities. This helps maintain a healthy sender reputation and ensures your emails consistently reach the inbox, fostering positive relationships with your subscribers and mailbox providers alike.
Yahoo and 
Microsoft (Outlook.com), participate in FBL programs. However, 
Gmail notably does not provide traditional FBLs, relying instead on data visible in their Postmaster Tools to indicate spam rates and other metrics. This distinction is crucial for ESPs and senders to understand when monitoring their complaint rates. You can learn more about this by checking which mailbox providers report spam complaints.
Google's Postmaster Tools provide general spam rate thresholds. Exceeding these thresholds can severely damage your sender reputation, making it much harder to reach the inbox. ESPs actively monitor these rates, as their overall infrastructure reputation is at stake. For more on this, read about what is considered a high spam rate.
