Oath, the advertising arm of Verizon — which owns both Yahoo and AOL — has been pitching advertisers a service that would allow them to peer into the email accounts of more than 200 million people.

The service benefits advertisers by allowing them to identify and segment potential customers by picking up on contextual buying signals, and past purchases. It’s a service that, according to Dough Sharp, Oath’s VP of data measurement and insights, allows anyone willing to spend the money to take a peek behind the curtain at commercial emails in any accountholders inbox — although, presumably, the data would be anonymized.

“Email is an expensive system,” Sharp said. “I think it’s reasonable and ethical to expect the value exchange, if you’ve got this mail service and there is advertising going on.”

Those paying Yahoo $3.49 a month for the premium, ad-free experience aren’t immune. The service still scans their mailbox for commercial mailings.

The system works much like advertisements on Facebook. Using a series of algorithms, the system spots potential targeting matches and places a piece of tracking code, a “cookie,” on a users computer. From there, advertisers are able to monitor, in some regards, what a user does online — including discerning whether that user visited the advertisers website days or weeks after seeing an ad, and whether they purchased anything while there.

It’s a system that’s fallen out of favor in recent years, especially among email providers. Gmail, the world’s most popular email provider — with 1.4 billion accounts — stopped scanning messages last year. Microsoft, the former leader in the email space, has never allowed advertisers to access email data.

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