Forget machines, even humans read your Gmail messages
According to a report by The Wall Street Journal, third-party app developers can read the e-mails of Gmail users who sign up for these email-based apps and services. They can access recipient addresses, timestamps, and entire messages. This is in complete contrast to Google's claims (it made last year) that it would stop its algorithms to scan the contents of Gmail users. Forget machines, even humans read your emails now.
You give them permission to read e-mails, but do you?
Notably, when you sign up for a separate e-mail based service, you do explicitly give consent to them to access this data. But at the time, you are only told that your e-mails will be monitored. It isn't clear that it would allow humans (and not just machines) to read your emails. In its defense, Google said that this permission is only given to vetted third-party developers.
Gmail's access settings allows app developers to see your e-mails
Some of those vetted third-party apps include e-mail managing firms Return Path and Edison Software. They have had access to thousands of e-mail accounts in the past. They also admit that they allow human engineers to view user e-mails so that the latter can train machine algorithms with regards to handling such huge amounts of data. Let that sink in!
Google claims it also denies Gmail access to some apps
Google's vetting process apparently involves checking if the app is correctly representing its company's identity, if it mentions in its privacy policy about monitoring e-mails, and if the data that the app is requesting is relevant to the service it provides.
Not clear how secure this system really is
So far, there has been no report of a third-party Gmail add-on developer having misused users' e-mail data. To be safe, you should check which apps have access to your Gmail account. Google added that its employees may also read e-mails but only in "very specific cases where you ask us to and give consent, or where we need to for security purposes."
Google outlines measures you can take to protect your data
Since the reports of the widespread yet discreet practice have surfaced, Google has published some preventive measures. Google suggests users thoroughly review the permissions screen of an email-based app and use its "Security Checkup" to see what permissions other apps have vis-à-vis their Gmail account.
