The chief executive of email unsubscription service Unroll.me has said he is "heartbroken" that users felt betrayed by the fact that his company monetizes the contents of their inboxes by selling their data to companies such as Uber.

Founded in 2011, the free web service allows users to unsubscribe en masse from mailing lists, newsletters and other email annoyances. To do so, it requires access to the users’ inboxes, and permission from them to scan the data for unsubscribe links.

But following an acquisition by shopping app Slice in 2014, Unroll.me developed a side-business: selling aggregated data about users to the very apps they were unsubscribing from.

The revelation came as part of a New York Times story about Uber, which was one of Slice’s big data arm Slice Intelligence’s customers: the cab app wanted to find out information about the corporate health of its key US rival, Lyft. The data Slice sells is anonymized -- customer’s names are not attached -- and it covers both Uber and Lyft ride receipts, but the company won’t confirm or deny its customer list.

Following the story, Unroll.me’s CEO and co-founder Jojo Hedaya wrote a corporate blogpost in which he expressed contrition. But while he said it was "heartbreaking", he was not talking about the sale of customer data: instead, he said he felt bad "to see that some of our users were upset to learn about how we monetise our free service."

He added: "the reality is most of us -- myself included -- don’t take the time to thoroughly review" terms of service agreements or privacy policies.