What is spam complaint rate?
Spam complaint rate is the share of recipients who marked your email as spam. It is the single most damaging email metric, because mailbox providers treat complaints as a direct vote that your mail is unwanted. Even a small number matters. Complaints are measured against tiny thresholds, and crossing them affects whether your future emails reach the inbox at all.
The formula.
Spam complaint rate = (spam complaints divided by delivered) times 100. The numbers are small, so the rate is usually shown to several decimal places. Providers like Gmail report this in their postmaster tools. Watch the figure they show you, since it is the one that affects your sending.
A worked example.
Of 2,000 delivered emails, 1 is marked as spam. The complaint rate is 1 divided by 2,000, times 100, which is 0.05%.
What is a good spam complaint rate?
Keep it under 0.1%. Mailbox providers including Gmail and Yahoo expect complaints below that line, and 0.3% is a hard limit that puts deliverability at serious risk. Unlike most metrics, there is little tolerance here, so the target is as close to zero as possible.
How to lower it.
Only email people who clearly opted in, make unsubscribing one easy click, and stop sending to recipients who have not engaged in a long time. Confirmed consent is the single biggest protection.