Complaint Rate
The percentage of recipients who mark your email as spam, a critical metric for sender reputation.
Definition
Complaint rate (also called spam complaint rate) measures the percentage of email recipients who click the 'Mark as Spam' or 'Report Spam' button in their email client. This is calculated as (number of complaints / number of delivered emails) × 100. It is one of the most important metrics for maintaining sender reputation and inbox placement.
Why It Matters
Spam complaints directly damage your sender reputation and deliverability. ISPs closely monitor complaint rates - Google recommends staying below 0.1% and warns above 0.3%. High complaint rates can lead to blocking, blacklisting, or severe throttling. Even a few tenths of a percent can significantly impact deliverability.
How It Works
When a recipient marks your email as spam, their mailbox provider records this and often reports it through a feedback loop to your ESP. These complaints affect your sender reputation score with that provider. The reputation impact is cumulative - sustained high complaints cause escalating deliverability problems.
Example
Understanding complaint rate impact:
Campaign of 100,000 emails delivered: - 50 complaints = 0.05% complaint rate (healthy) - 100 complaints = 0.1% complaint rate (at threshold) - 300 complaints = 0.3% complaint rate (danger zone) - 500 complaints = 0.5% complaint rate (deliverability crisis)
Major ISPs may start throttling at 0.1% and blocking at 0.3%+
Best Practices
- 1Keep complaint rate below 0.1% (Google's threshold)
- 2Make unsubscribe easy and prominent to reduce spam complaints
- 3Only email people who explicitly opted in
- 4Set proper expectations during signup about email frequency
- 5Honor unsubscribe requests immediately