Unroll.me, a free service to unsubscribe from email lists, can scour people’s inboxes for receipts from services like Lyft and then sell the information to companies like Uber. The data is anonymized, meaning individuals’ names are not attached to the information, and can be used as a proxy for the health of a rival.

After the revelation, angry users demanded that Unroll.me explain why the company had gone into their inboxes and betrayed their trust. Late on Sunday, Jojo Hedaya, the chief executive of Unroll.me, apologized in response to the surprised reaction to a practice that he said the company had been open about in the past.