Open Rate.

The open rate is the best-known metric in email marketing. It is calculated on delivered emails, not on those sent: if you send 1,000 emails, 980 are delivered and 300 are opened, the open rate is around 31 per cent (300 ÷ 980). Some tools also report unique opens, meaning recipients who opened at least once, and total opens, which count repeat opens. For comparisons, always use the same variant.

An open is measured through a tiny, invisible image in the email's HTML. When the email client loads this image from the sending service's server, it counts as an open. That creates two sources of error. People who block images or read in plain-text view open the email without being counted. And clients that preload images automatically are counted although no human has seen the email.

The second error has been the bigger one since 2021. With iOS 15 and macOS Monterey, Apple introduced Mail Privacy Protection (source: Apple, WWDC announcement, June 2021). When it is active, Apple preloads the images of all emails via its own servers, regardless of whether the email is read. To the sender, this looks like an open. The more recipients use Apple Mail, the more the open rate is inflated, and open time and location are no longer accurate either.

The open rate therefore remains useful for rough comparisons within the same list, for example to see whether one subject line performs noticeably worse or whether emails are suddenly landing in spam, see email deliverability. For decisions, metrics that measure real action are better: the click rate (see CTR), replies, unsubscribes and above all enquiries or purchases from an email, in other words the conversion rate. If you evaluate A/B tests on open rate, look at Apple recipients separately or switch to clicks.