Starting in iOS 15, Apple prompts users using Apple Mail to turn on a privacy setting that causes Postmark to detect false-positive opens. When turned on by an iOS user, Apple Mail will pre-load and cache images by routing messages through an Apple hosted proxy server. This means if there’s a tracking pixel in the content for collecting open data, the sender will instead see an artificially inflated open rate with the proxy’s data instead.
Before diving into this feature, a quick refresher on how Postmark's open tracking works. For messages with open tracking turned on, Postmark inserts a 1 x 1 invisible pixel into those messages. When that image is downloaded(mail client opening the message), Postmark marks the message as opened. How Postmark tracks opens is a standard practice that other email providers also use.
The new iOS feature is called Mail Privacy Protection (MPP). This setting is prompted to users opening Apple Mail for the first time after updating to iOS 15 — They're asked if they'd like to"Protect Mail activity" or not. When turned on, message content is routed through an Apple proxy server. Apple downloading messages to their proxy server server will prompt a false-positive open event in Postmark, as it triggers our tracking pixel.
When the recipient opens the message, Apple then shows the cached content of the message to the recipient, rather than downloading the pixel from Postmark.
This means it is possible to see inflated open events from recipients using Apple Mail if a user turns on this feature. This privacy step by Apple Mail is a reminder open tracking is not 100% accurate, as there are other known limitations with open tracking. While open rates are sometimes a good proxy to help ascertain the deliverability of your messages and see if a message connects with your audience, there are other ways to help with that. We encourage you look for ways to engage your recipients as a willing participant, such as asking for feedback on your content or letting them reply to your messages directly to gauge the performance of a message.
Apple Mail works with different email accounts like Gmail, Yahoo, Hotmail, and most company domains so it's possible to see this happen when the recipient isn't using an Apple hosted domain for their email address.