The Wall Street Journal, which was the first to point this out, called it "tech's dirty secret" that has been kept under wraps for a long time.
Hundreds of app developers electronically "scan" inboxes of the people who signed up for some of these programs, and in some cases employees do the reading, the paper reported.
The Wall Street Journal's Douglas MacMillan reports Google promised to stop the practice because it wanted users to "remain confident that Google will keep privacy and security paramount".
Some of these companies train software to scan the email, while others enable their workers to pore over private messages, the report says.
Google, a unit of Alphabet Inc., says it provides data only to outside developers it has vetted and to whom users have explicitly granted permission to access email.
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But Holt, the customer whose grass was being trimmed by the kids, thought calling the police over this was just ridiculous. Fields said he's trying to save the money he earns to buy new equipment and grow his business.
One such company is Return Path, which collects data for marketers by scanning the inboxes of more than two million people who have signed up for one of the free apps in its partner network using a Gmail, Microsoft or Yahoo email address. This is in contrast with what Google promised previous year, where it said that it would stop reading its users email messages, which might be true, but it has done very little to stop other partner organisations from doing so.
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Google is allowing app developers to sift through your Gmail account. The search giant even rolled out new features for Android users that give them better control over privacy settings in their Gmail accounts. He says engineers at eDataSource occasionally reviewed emails when building and improving software algorithms.
Gmail has almost 1.4 billion users globally - more users than the next 25 largest email providers combined.
It pointed the BBC to its developer policies, which state: "There should be no surprises for Google users: hidden features, services, or actions that are inconsistent with the marketed goal of your application may lead Google to suspend your ability to access Google API Services".