Open Rate

Open rate divides the number of opens by the number of delivered emails. Senders almost always use unique opens (one per recipient) rather than total opens, so a reader who opens the same message five times still counts once:

There is no native “was this opened” signal in email, so senders embed a tiny 1 by 1 pixel image, hosted on a tracking server, into the message. When the recipient’s client renders the email, it requests that image, and the server logs the request as an open along with a timestamp and rough location.

The method was always imperfect: a recipient who reads with images turned off never loads the pixel and never registers as an open, so open rate has always undercounted to some degree. The bigger problem is the opposite, and it is recent.

In September 2021, with iOS 15, Apple launched Mail Privacy Protection (MPP). For any recipient who enables it in the Apple Mail app, Apple pre-fetches all remote content (including the tracking pixel) through its own proxy servers as soon as the message arrives, regardless of whether the person ever opens it. To the sender, every one of those recipients looks like an open.

The effect is large. Apple Mail accounts for a majority of recorded email opens, and analyses put the resulting inflation in the range of roughly 15% to 35% for senders with a meaningful Apple audience. Newsletters that genuinely opened at, say, 28% routinely now report well above 50%. The opens are real requests, but they are machine requests, not human reads. MPP also masks IP and approximate location, so open-based geolocation and precise open-time data are no longer dependable.

Open rate is not useless, but it can no longer be taken at face value. Use it as a relative trend within a stable audience rather than an absolute truth, and lean on signals MPP does not touch:

• Clicks and click-through rate. A click requires a deliberate human action, so it survives MPP intact and is the most reliable engagement measure now.
• Click-to-open rate (CTOR). Clicks divided by opens; useful for content quality, though the open denominator is itself inflated, so read it cautiously.
• Conversions and replies. The downstream actions that actually matter to the business and cannot be faked by a proxy.

Because mailbox providers judge engagement on real interaction, optimising for genuine clicks and conversions also protects sender reputation in a way that chasing an inflated open number never could.