What is Delivered, sent, and opened?

Every newsletter rate is a fraction, and four different counts compete to be its denominator: list size, emails sent, emails delivered, and emails opened. Sent minus bounces is delivered. Delivered minus the unopened is opened. Each step down shrinks the denominator and inflates the rate.

This is how the same send honestly produces a 2.8% rate and a 7.5% rate at once. Neither number is wrong; they answer different questions. The habit that keeps your metrics comparable over time is simple: pick a denominator deliberately and hold it constant.