Glossary
Mail Privacy Protection
Also called MPP, Apple Mail Privacy Protection.
Last reviewed 18 September 2026
What is Apple 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.
Apple documents the effect plainly: the feature stops senders knowing when mail has been opened. On any list with a substantial share of Apple Mail readers, and most consumer lists have one, the open rate rises when nobody reads anything, so it is not noisy data but wrong in a consistent and flattering direction.
It also hides the reader’s address from the fetch, so the location and time-of-open inferences drawn from a tracking pixel are describing a proxy. What is left are the signals that require a decision by a person: a click, a reply, a confirmation.
See also
- Open rate
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.
- 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.
- Completion rate
A completion rate is the share of confirmed subscribers who reach the end of a course, and for a five-day course measured by a click on the last day it commonly runs between one in ten and one in four, with longer courses lower.
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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