Glossary
Open rate
Last reviewed 18 September 2026
Why is open rate unreliable?
An open rate is the share of delivered messages in which a tiny remote image loaded, used as a proxy for a human having looked, and it is unreliable because that image is fetched by proxies and caches that are not readers and never fetched by clients that block images.
The false positives and the false negatives arrive in proportions that differ by provider and change without notice. Gmail fetches and caches remote images through its own servers, Apple Mail Privacy Protection fetches them on the reader’s behalf whether or not the message was opened, and any client with images off registers nothing at all.
A sender who prunes a list by open rate is therefore pruning partly at random. There is a second objection with nothing to do with accuracy: the pixel puts a third-party host and a remote image load into every message, and an invisible image reporting a reader’s behaviour is processing personal data that has to be disclosed and justified like any other.
See also
- Mail Privacy Protection
Mail Privacy Protection is the Apple Mail feature that loads a message’s remote content through a proxy whether or not the reader opened it, which is what stopped an open rate from measuring anything about a person.
- Click-asserted progress
Click-asserted progress is measuring how far a reader got through a course by acts that required a decision from a person, such as a click on a link in the last lesson, rather than by an open, which is an image load a machine can perform.
- Deliverability
Deliverability is the share of the mail you send that reaches the inbox of the person you addressed it to, as distinct from delivery, which is only the share the receiving server accepted rather than bounced.
Read next
This term is covered properly in Email deliverability for small senders: authentication and reputation, which is a longer piece on the subject.
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