According to the report, companies like Return Path Inc., which collects marketing data from people who sign up for one of the company's free apps, can scan and analyze about 100 million emails each day. The WSJ also says that while computers do most of this work, human employees read about 8,000 emails to help train the software. It's not just one company, either. Employees of Edison Software apparently reviewed the emails of hundreds of its users to make a new feature for its mobile app that reads and organizes your email.
Both companies told the WSJ that the practice is covered under their user agreements and that its employees have strict rules about what they can and can't do with the emails they read. While the WSJ found no evidence that these companies misused the data they collected, it's still a blow to user confidence, especially as companies like Google and Facebook keep saying that they're protecting your data.
Update: Google also responded with a post on its blog, highlighting the same things mentioned in its response above.