Apple iOS 15 and Mac OS Monterey - and newer versions - come with a setting called Mail Privacy Protection for Apple Mail.

With this setting enabled it's not possible to tell whether, where or when an email was opened because the tracking image that open tracking relies on (a small, transparent pixel that's not visible for contacts) is preloaded even if contacts don't actually open their emails.

This results in unreliable open tracking for your contacts who use Apple Mail to read their emails with Mail Privacy Protection enabled.

Delivery of emails and click tracking are not affected.

Related article: Open tracking in emails

Email campaign reports

Open rates in email campaign reports may be over inflated depending on how many contacts on your list use Apple Mail with the Apple Mail Privacy Protection enabled.

AB testing

Since AB testing is based on email opens, its results will be less reliable. This depends on the number of contacts on your list who use Apple Mail with the Apple Mail Privacy Protection enabled.

Segmentation

When you filter your list for campaign activity using "opened" or "didn't open" as a condition the results may be inaccurate - again, depending on how many contacts on your list use Apple Mail with the Apple Mail Privacy Protection enabled.

Re-sending emails to non openers

There may be contacts who won't receive your reminder email if you re-send it to anyone who didn't open the original because the new Apple privacy setting will make it look like the original email was opened.