Open Rate
The percentage of delivered emails that were opened by the recipient.
Open rate is the percentage of recipients who opened a campaign. Most platforms calculate it as unique opens divided by delivered emails; some divide by emails sent, so check your tool's definition before comparing numbers. An open is recorded when a tiny tracking image in the email loads, which makes the metric an estimate rather than a count of people who read the message.
How to read open rate today
Use unique opens, not total opens, and compare the same kind of email over time on the same platform. In 2021 Apple introduced Mail Privacy Protection, which stops senders from knowing when a user opens an email and masks the user's IP address. For people who turn it on, Apple Mail loads the tracking image in the background, so opens are recorded whether or not the person read the email. Some platforms label or filter these machine opens; check whether yours does.
The error runs the other way too: mail apps that block images by default undercount real opens.
Worked example
A campaign sent to 10,000 people with 200 bounces has 9,800 delivered. If 3,920 different people register an open, the unique open rate is 3,920 ÷ 9,800 = 40%. Reported against emails sent instead, the same campaign shows 39.2%. Neither number tells you how many of those opens came from Apple Mail preloading the tracking image.
Common open rate mistakes
Picking subject-line winners or send times from opens alone when much of your list uses Apple Mail. Triggering automations or "engaged" segments on opens without also checking clicks or purchases. Comparing your open rate with a published benchmark that used a different denominator or counted machine opens. Clicks, replies and conversions are more reliable signals of interest.