What is considered a high spam rate that leads to ESP blocking and how should complaint rates be calculated?
Michael Ko
Co-founder & CEO, Suped
Published 6 Jul 2025
Updated 16 Aug 2025
7 min read
Email deliverability is a complex landscape, and understanding your spam rate is crucial for ensuring your messages reach the inbox. A high spam rate not only impacts your sender reputation but can also lead to email service providers (ESPs) and internet service providers (ISPs) blocking your emails entirely. This means your legitimate communications might never see the light of day, significantly harming your outreach efforts and business operations.
The challenge lies in knowing what constitutes a high spam rate and, more importantly, how to accurately calculate your complaint rates to identify potential issues before they escalate. Misinterpreting these metrics can lead to misguided strategies and persistent deliverability problems.
Understanding high spam rates
What is a high spam rate?
A high spam rate is generally defined as the percentage of your emails that recipients mark as spam or junk. This metric is a strong indicator to mailbox providers of the quality and relevance of your email content and your sending practices. Different providers and industry standards have varying thresholds, but exceeding these can quickly damage your sender reputation and lead to serious deliverability issues.
Many in the industry consider anything above 0.1% to be a red flag. This means if 1 out of every 1,000 emails sent results in a spam complaint, you are at a potentially problematic level. Major providers like Gmail and Yahoo, which have recently introduced stricter sending requirements, advise senders to aim for significantly lower rates, with a hard threshold at 0.3%. Exceeding 0.3% (more than 3 complaints per 1,000 emails) is generally considered an excessive level of spam complaints and can lead to significant blocking.
Failing to keep your spam rate low can result in your IP address or domain being added to a blocklist (or blacklist), making it nearly impossible to reach your audience's inboxes. It's not just about bulk senders, even transactional emails can be affected. Understanding what is considered an acceptable email complaint rate benchmark is the first step towards healthy email deliverability.
Key spam rate thresholds
Keeping your spam complaint rate below critical thresholds is essential for maintaining good sender reputation. Even minor increases can trigger filtering by mailbox providers. For most inbox providers, anything under 0.1% (1 complaint per 1,000 emails) is generally considered acceptable. However, leading providers are even stricter.
Mailbox Provider
Acceptable Spam Rate (%)
Blocking Threshold (%)
Gmail
< 0.1%
> 0.3%
Yahoo/AOL
< 0.1%
> 0.3%
Microsoft (Outlook/Hotmail)
~ 0.2%
> 0.2%
General Industry Standard
< 0.1%
> 0.1%
How complaint rates are calculated
Calculating email complaint rates accurately
Accurately calculating your spam complaint rate is more nuanced than simply dividing total complaints by total emails sent. Many people make this mistake, leading to a vastly under-reported rate. The key is to understand that not all mailbox providers offer feedback loops (FBLs), which are systems that notify senders when their emails are marked as spam.
The more precise way to calculate your complaint rate is by dividing the number of complaints received by the number of emails sent to domains that actually support FBLs. This provides a more realistic view of your complaint rate, as it accounts for the visibility you have into these reports. Even then, the calculation can be skewed, for example, if a significant portion of your mail is being delivered to the spam or bulk folder and therefore not generating FBLs.
Remember that B2B sending often has fewer FBLs available, making accurate complaint monitoring even more challenging. You can learn more about how to monitor complaint rates using Google Postmaster Tools, Yahoo FBL, and your ESP data.
Incorrect method
Formula: Complaints received / Total emails sent x 100
Under-reporting: This method can falsely suggest a lower complaint rate, leading to complacency.
Blind spots: It fails to account for mailbox providers that do not send FBLs, creating hidden issues.
Corrected method
Formula: Complaints received / Emails sent to FBL-supporting domains x 100
More accurate: Provides a more realistic view of your complaint rate where data is available.
Better insight: Helps pinpoint issues within segments where FBLs are active.
Complaint rate calculation
(Complaints Received / Emails Sent to FBL-Supported Domains) * 100
The ripple effect of high spam rates
The ripple effect of high spam rates
A high spam complaint rate has a cascading effect on your email deliverability. The most immediate consequence is a damaged sender reputation. Mailbox providers and ESPs use complex algorithms to evaluate your sending behavior, and frequent spam complaints signal that your emails are unwanted. This can lead to your emails being filtered directly into spam folders, throttled, or even outright blocked, impacting your ability to reach customers and prospects.
Beyond deliverability, a poor sender reputation can result in your IP address or domain being added to various blocklists (also known as blacklists). Once on a blocklist, your emails may be rejected by a wide range of recipients, even those not directly associated with the complaint. Recovering your domain reputation from a low standing can be a lengthy and challenging process, requiring consistent effort to demonstrate positive sending habits. Read our in-depth guide to email blocklists for more.
The ultimate impact is on your return on investment from email marketing. If your emails aren't reaching the inbox, your open rates, click-through rates, and conversion rates will naturally suffer. It's a clear signal that recipients are not finding your content valuable or relevant, or perhaps they didn't explicitly opt-in to receive your emails in the first place. You can read more about email deliverability issues in our latest report.
Strategies to maintain a healthy complaint rate
Strategies to maintain a healthy complaint rate
Proactively managing your spam complaint rate involves a combination of best practices focused on list quality, content relevance, and user experience. Start with ensuring that you only send emails to individuals who have explicitly opted-in to receive communications from you. Implementing a double opt-in process is a highly effective way to confirm subscriber intent and reduce the likelihood of spam complaints from disengaged users.
Regularly cleaning your email list to remove inactive subscribers, hard bounces, and known spam traps is also essential. This ensures you're sending to an engaged audience, which naturally reduces complaints. Focus on delivering valuable and relevant content that your subscribers expect. Misleading subject lines, irrelevant content, or sending too frequently can all contribute to recipients marking your emails as spam. Understanding how you get spam complaints is key to avoiding them.
Finally, make it easy for subscribers to unsubscribe if they no longer wish to receive your emails. A clear, one-click unsubscribe link in your email footer can prevent a recipient from resorting to the report spam button. By prioritizing subscriber experience and list hygiene, you can significantly reduce your spam complaint rate and improve your overall email deliverability. For more insights, refer to our guide on why your emails are going to spam.
Views from the trenches
Best practices
Actively monitor all available feedback loops (FBLs) from mailbox providers to capture complaint data. This is crucial for early detection.
Always use a double opt-in process for new subscribers to ensure explicit consent and reduce unwanted emails.
Segment your audience effectively to send highly relevant content, improving engagement and reducing complaint likelihood.
Regularly clean your email lists by removing inactive subscribers and hard bounces to maintain list quality.
Common pitfalls
Calculating complaint rates based on total emails sent instead of emails sent to FBL-supporting domains, which under-reports the issue.
Ignoring even a single complaint, as ESPs can take compliance actions for isolated incidents, not just percentages.
Assuming that a lack of FBLs in B2B sending means zero complaints, missing potential hidden deliverability issues.
Not accounting for mail that goes directly to the spam folder, as this traffic will not generate FBL reports, skewing your metrics.
Expert tips
No single metric is perfect in isolation, look at how all your email metrics relate to one another to understand the full story of your deliverability.
If your emails are consistently going to the bulk or spam folder, your FBL rate will be artificially low because FBLs are often not generated for messages filtered there.
Be aware that some ESPs might take compliance action even for a very small number of complaints, not necessarily waiting for a high percentage.
Focus on the actual inbox placement rate rather than solely relying on reported complaint rates from FBLs, as not all complaints are reported.
Marketer view
Marketer from Email Geeks says a complaint rate as high as 0.1-0.05% indicates severe reputation damage.
2023-02-22 - Email Geeks
Marketer view
Marketer from Email Geeks says Hotmail's blocking threshold appears to be around 0.2%.
2023-02-22 - Email Geeks
Maintaining optimal deliverability
Maintaining optimal deliverability
Understanding and managing your spam complaint rate is fundamental to achieving high email deliverability. By accurately calculating this metric and adhering to industry benchmarks, you can proactively identify issues and implement strategies to protect your sender reputation. Consistently low complaint rates signal to mailbox providers that your emails are valued, ensuring they reach the intended recipients' inboxes and support your overall email strategy.
Gmail and 
Yahoo, which have recently introduced stricter sending requirements, advise senders to aim for significantly lower rates, with a hard threshold at 0.3%. Exceeding 0.3% (more than 3 complaints per 1,000 emails) is generally considered an excessive level of spam complaints and can lead to significant blocking.
Microsoft (Outlook/Hotmail)
