What you need to know:
Many of the top personal email service providers scan user’s emails. The purpose of these scans can vary, including using the data to enhance services, provide product personalization, detect fraud, create auto-generated replies or in some cases explicitly for targeted advertising. Even if the service isn’t directly using this data for targeted advertising, the data is stored in a server somewhere and likely still contributes to these companies’ profile and understanding of you.
In addition to the service provider, some third-party apps that provide additional services for your inbox (like tidying up your inbox, package tracking, price comparison, etc), also scan your inbox and sell that data to marketers and financial institutions. Most of this data collection to date has been focused on purchase history, but as artificial intelligence becomes more sophisticated, the data these companies can gleam from an inbox will exponentially increase.
Finally, most marketing and promotional emails are embedded with trackers that allow the sending company to know information including when and how often an email is read, whether links were clicked, what device was used, as well as general location where the email was opened. Some of this technology is now available to individuals and private citizens as well, beyond just brands, retailers and other marketers.
How Google had stopped using Gmail data directly for advertising but still scans inboxes for other purposes
How Yahoo! does explicitly scan inboxes for targeted advertising
Edison an email app, in addition to third party apps like Cleanfox and Slice sell data from inboxes
Read this story about how one fan used trackers on Jay Z
What to do:
To block trackers placed within emails see this guide here.