The complaint rate: the most important figure

This is the measurement mail providers take most seriously, and the one most senders never look at.

What it is

The proportion of recipients clicking "report spam". The large providers expect this figure to stay extremely low — well below a tenth of a per cent.

Why it weighs so heavily

A complaint is a direct human signal: this person did not want your message. No other indicator carries that weight.

Where complaints come from

An unsubscribe link that is hard to find, a higher frequency than announced, or recipients who do not remember signing up. Rarely from the content itself.

How to lower it

Make unsubscribing easier than complaining. It is counter-intuitive and it is the most effective measure there is.

Monitor it

A sudden rise after a campaign is a warning. Waiting for the block means responding a month too late.