Definition
Complaint rate is the percentage of delivered emails that are marked as spam or junk by recipients. It is calculated by dividing the number of spam complaints by the number of emails delivered, then multiplying by one hundred.
Complaint rate is one of the most important signals mailbox providers use to judge sender reputation. A low complaint rate indicates that recipients want the email. A high complaint rate suggests the sender is sending unwanted mail, which can lead to spam folder placement, throttling or blocking.
Industry benchmarks vary, but a complaint rate below 0.1 percent is generally considered healthy. Rates above 0.3 percent are a warning sign, and rates above 0.5 percent can cause serious deliverability problems. Improving complaint rate usually requires better list quality, clearer expectations, easier unsubscribe and more relevant content.
Formula
Complaint Rate = (Spam Complaints / Emails Delivered) × 100
| Variable | Description |
|---|---|
| Spam Complaints | Number of recipients who marked the email as spam or junk |
| Emails Delivered | Total number of emails accepted by mailbox providers, excluding bounces |
Best Practices
- Use confirmed opt-in to ensure subscribers expect your emails.
- Set clear expectations about content and frequency at signup.
- Make unsubscribing easy and process requests immediately.
- Segment by engagement and suppress inactive subscribers.
- Monitor complaint rates by campaign, domain and segment to identify problems early.
Related Glossary Terms
A/B Testing
A/B testing in email marketing is the practice of sending two variations of an email to a small sample of your list to determine which version performs better before sending the winner to the remaining subscribers.
Abandoned Cart Email
An abandoned cart email is an automated message sent to customers who added items to their online shopping cart but left without completing the purchase. It is one of the highest-converting email types in ecommerce.
Abuse Complaint
An abuse complaint is a report from a recipient who marks an email as spam, which negatively affects sender reputation and deliverability.
ARPU (Average Revenue Per User)
ARPU (Average Revenue Per User) is a metric that measures the average revenue generated per email subscriber over a specific period, used to evaluate list value and campaign effectiveness.
Attention Rate
Attention rate is the percentage of email opens that last longer than 5 seconds, distinguishing genuine reads from passive opens, preview-pane views, or Apple MPP auto-loads.
Average Order Value in Email
Average order value in email is the average amount spent per transaction from recipients who clicked through from an email campaign.
Frequently Asked Questions
Divide spam complaints by delivered emails and multiply by 100. For example, 5 complaints from 10,000 delivered emails is a 0.05 percent complaint rate.
Below 0.1 percent is healthy. Above 0.3 percent is concerning, and above 0.5 percent can cause serious filtering.
Common causes include poor list quality, misleading subject lines, unexpected frequency, irrelevant content and difficult unsubscribe processes.
Yes. Providers calculate and weigh complaints differently. Some are more sensitive to sudden spikes or complaints from new subscribers.
Send a re-engagement campaign to inactive subscribers, remove non-engaged addresses, simplify unsubscribe and audit your signup sources.