What is Spam rate (complaint rate)?
The share of your delivered mail that recipients mark as spam; Gmail says keep it below 0.3% and ideally under 0.1%.
When a recipient clicks "Report spam", the provider counts it against your domain. Google's sender guidelines require bulk senders to keep the spam rate shown in Postmaster Tools below 0.3% and recommend staying under 0.1%. A working one-click unsubscribe, a visible opt-out line, a sequence that stops on any reply and a company-wide pause on "please stop" all exist to keep this number down — an easy way out is cheaper than a complaint.