http://www.wired.co.uk/article/gmail-security-checkup-apps-data
Quoting:
"Google’s business is in data. The data it gathers from each time its products are used – from maps to search – allows it to personalise the adverts it sells. The more targeted an advert is, the more money can be made from it. But not all data.
In 2017 Google drew a line in the sand: emails are off limits. The firm announced after a decade of scanning and reading emails to help with personalisation it would stop the practice. User information should "remain confidential", Google said at the time.
There has been an exception to the rule. Companies that build handy add-ons for Gmail, which allow emails to be scheduled or calendar information to be extracted, have been able to read people's emails. As first reported by the Washington Post, the developers of hundreds of apps have been able to read people's emails, including entire messages, who they were sent to and other private details.
Human engineers from firms such as Return Path (an email marketing company) and Edison Software (the creators of an email app), the Wall Street Journal reported, have been able to read the emails of those that use their programs. This reading hasn't just been limited to machines but also includes humans who have had manual access. The third-party developers have used this access to improve their own software."
