Tracking when someone opened an email and what was read is something many businesses and advertisers rely on for their marketing efforts. Additionally, there are email clients designed to let users know when the emails they sent were opened.
Much of this tracking is facilitated by remote images that load when viewing an email. Some of it is even more insidious, as advertisers use invisible pixels. Tracking pixels are hidden graphics that users may not see in an email, but their email client loads them, allowing senders to collect data from recipients.
Data protection in emails can be controlled at any time
Senders can see that recipients have opened an email to get other information, such as the IP address. With iOS 15, iPadOS 15 and macOS Monterey, Apple puts an end to email tracking with a range of email security features. Mail Privacy Protection is not enabled by default, but Apple will highlight it as an option when users update to iOS 15 or iPadOS 15.

The whole thing can also be switched on and off at any time under "Settings" -> "Mail" -> "Privacy" -> "Protect Mail Activity". If you want to control the whole thing under macOS Monterey, you can do so in the Mail app under Settings in the "Privacy" section.

When enabled, Mail Privacy Protection hides your IP address and loads all removed content privately in the background by routing it through multiple proxy services and randomly assigning an IP address. Here's how Apple describes the new security feature:
iOS 15 & macOS Monterey: Apple's statement in full
Emails you receive may contain hidden pixels that allow the sender of the email to learn information about you. Once you open an email, information about your mail activity may be collected by the sender without any transparency or control over what information is shared. Email senders may learn when and how often you opened their email, whether you forwarded the email, your Internet Protocol (IP) address, and other data that can be used to build a profile of your behavior and learn your location. When you enable this feature, Mail Privacy Protection helps protect your privacy by preventing email senders, including Apple, from learning information about your mail activity.
It continues:
When you receive an email in the Mail app, Mail Privacy Protection by default downloads removed content in the background instead of loading it when you open the email—regardless of how you interact with the email or not. Apple doesn't learn any information about the content. Additionally, all content downloaded by Mail is routed through multiple proxy servers to prevent the sender from learning your IP address. Instead of passing along your IP address, which could allow the email sender to learn your location, Apple's proxy network randomly assigns an IP address that matches only the region where your device is located. As a result, email senders only receive general information and no information about your behavior. Apple has no access to your IP address.
Background tracking is history
Previously, users could block email trackers by blocking the loading of removed content in the Mail app on iOS and macOS. Apple's new feature is better, however, as users can still view all email content as usual while Mail privacy works in the background without any visual impact. Of course, email senders can still monitor your behavior with tracking links - as long as you interact with them. But background tracking is definitely a thing of the past with the new feature. (Photo by hadrian / Bigstockphoto)