Uber’s bad PR is growing -- and spreading to its business partners.

On Monday, internet users were in an uproar after a New York Times article revealed that a popular email management service sold data on its users’ ride-hailing information to Uber.

The New York Times profile of Uber’s CEO, Travis Kalanick, noted: “Using an email digest service it owns named Unroll.me, Slice [Intelligence] collected its customers’ emailed Lyft receipts from their inboxes and sold the anonymized data to Uber. Uber used the data as a proxy for the health of Lyft’s business.”

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The revelation was ironic, since Unroll.me purports to give users back control of their inboxes by helping them unsubscribe from newsletters. The backlash was swift. “Stop using unroll.me right now, it sold your data to Uber,” wrote The Intercept’s Sam Biddle. Tech writer Andy Baio reminded Twitter that “giving startups silent, perpetual access to your Gmail is a terrible idea.”

After Slice’s CEO wrote a subdued apology, saying “it was heartbreaking to see that some of our users were upset to learn about how we monetize our free service,” users skewered him in the comments and demanded their records be destroyed.

Slice Intelligence, which owns Unroll.me, confirmed to CBS News that it sold anonymized data -- meaning that the information did not include users’ names. The service analyzes an email’s header and subject information to determine if the person is receiving a receipt, and from what company or retailer, said Jaimee Minney, vice president of marketing and public relations for Slice. If a receipt is attached, the record is pulled and analyzed, she said.

Minney added that receipts regarding transportation purchases are tracked more than other types of transactions.

“We do track the whole travel sector --- for those guys we pull things like ‘What was the fare? Which market did it originate in?” she said in a phone call Monday afternoon. That information is used to estimate market share and other competitive information, she noted.

She also denied allegations, posted in a tech discussion forum, that the service kept copies of users’ emails on poorly secured servers. That rumor, along with certain executives’ reputation for dishonesty, was enough to keep another company from purchasing Unroll.me, wrote the poster, Karl Katzke.

“That post is completely untrue,” Minney said.

In an email Monday evening, Minney clarified the company’s email retention policy.

“We only store the HTML headers and promotional emails that are in people’s roll-ups (for seven days), so they can be accessed retroactively,” Minney said. “We do not store personal emails at all. Our security protections are all using industry best-practices.”

It’s unclear, however, what Unroll.me’s email retention policy is, with some Twitter exchanges indicating it has a seven-day policy.

The extent of users’ anger seems to have caught Slice off-guard, and it’s no surprise. Monetizing users’ data is how companies do business, down to and including Google itself. The email and web giant adjusts what it shows users in YouTube and search based on their email activity, and allows companies to target users based on where they share their email, The Verge reported.

Nonetheless, the backlash is a reminder that people generally don’t like being tracked, as they say in study after study. Periodic public outrage, say privacy advocates, is the expected result of a relationship in which service providers increasingly have outsized information and power.

“It’s a fallacy to think that [people] are willingly trading their information,” Jeff Chester, executive director of the Center for Digital Democracy, told CBS News earlier this month. “They simply feel as if they have no control and no choice, and that’s a pretty accurate feeling.”