An open is not a report from the reader. It is an image request. Anything that fetches images produces one. That single design choice is behind every limit on the number.
How is an open actually measured?
By loading a small image, which is the whole reason the metric is fragile.
An open is recorded when a mail client fetches a transparent pixel embedded in the HTML part of the message. There is no other mechanism. No mail client sends a signal saying a person read something. Bird's open and click tracking describes the instrument: a pixel is injected, and an email.opened event is recorded when it is fetched.
Anything that fetches images produces an open, whether or not a human was involved. Anything that does not fetch images produces no open, whether or not a human read the message.
What did Apple Mail Privacy Protection change?
It made the fetch deliberate and unconditional.
Apple's iPhone User Guide says the feature exists to stop a sender learning whether a message was opened:
In the Mail app, turn on Mail Privacy Protection to make it harder for senders to learn about your Mail activity. Mail Privacy Protection hides your IP address so senders can’t link it to your other online activity or determine your exact location. It also prevents senders from seeing if you’ve opened the email message they sent you.
Apple's announcement names the mechanism it defeats:
In the Mail app, Mail Privacy Protection stops senders from using invisible pixels to collect information about the user. The new feature helps users prevent senders from knowing when they open an email, and masks their IP address so it can’t be linked to other online activity or used to determine their location.
Hiding an open from the sender does not mean blocking the pixel, which would produce no open and reveal the setting. The pixel is fetched, for messages the person never looked at. The resulting open is indistinguishable from a real one unless you look at how it was fetched. Gmail's image proxy fetches images on the recipient's behalf too, with the same effect on the number.
Can the inflation be filtered out?
Partly. One direction of the error can be removed and the other cannot.
Bird marks proxied opens with is_prefetched: true and computes the dashboard open rate from unique non-prefetched opens only, leaving both figures available through the stats API. So the reported rate is not the raw one. The gap between the two is visible rather than assumed.
Filtering does not make opens exact. A recipient who blocks images or uses a text-only client reads the message without registering an open, which the Apple Mail Privacy knowledge-base page states directly.
So the error runs both ways. Privacy features inflate the number and blocked images deflate it. The two do not cancel. The balance between them differs by audience: a developer newsletter and a consumer promotion have very different populations of image-blocking readers.
Is an open a signal to anyone else?
Not directly. A mailbox provider weighs its own observations, not your pixel.
Engagement-based filtering is real. What a provider weighs is behavior inside its own product. It does not read your open rate. Your open rate does not read its signal.
Spamhaus, which publishes email blocklists, makes the point about the sender's side of this:
The fact that an email is opened or a link is clicked does not mean consent has been given.
Anti-abuse systems interact with mail as well. So a fetch is not consent, not attention, and not necessarily a person.
What should I use it for?
Use open-rate changes to choose an investigation. Confirm the cause with delivery, placement and completed-action data before changing a recipient's journey.
Open-rate comparisons can support these investigations:
- A relative change over time. Compare the audience, client mix and tracking settings before attributing a change to recipient behavior.
- A controlled variant comparison. Assign comparable recipient groups and keep the sending window and tracking settings consistent. Check client mix and privacy effects in each group; a shared audience alone does not establish equal distortion.
- A provider-level investigation. Compare placement estimates and delivery events when opens change. The open count alone cannot distinguish placement, content, tracking or audience changes.
Two uses that do not:
- Reporting an absolute figure as engagement. The number does not measure readers and should not be presented as one.
- Deciding whether to keep mailing one person. A per-recipient decision built on opens retires people who read your mail with images off.
Clicks work without image loading, but automated systems can also request links. Click-through rate measures those recorded requests. For a purchase campaign, compare completed purchases for the same audience and period; clicks alone cannot establish that result.
In short
An open is an image load, not a reading.
Open tracking works by embedding a pixel, so anything that fetches images produces an open and anything that blocks them hides one.
Apple's Mail Privacy Protection loads the pixel deliberately.
Apple's own description says the feature prevents senders from seeing whether a message was opened. It does that by fetching regardless.
The number is inflated and deflated at the same time.
Privacy proxies push it up while blocked images and plain-text readers push it down, so the error does not cancel in a knowable direction.
It survives as a trend, not as a measurement.
A relative change identifies something to investigate. Check tracking conditions, delivery and placement before attributing it to reading.