A federal judge ruled Thursday that Google may be violating wiretap law when it scans the e-mails of non-Gmail users, allowing a lawsuit against the company to move forward.

In her ruling, Judge Lucy Koh of the Northern District Court of California, questioned the clarity of Google’s privacy policy in explaining how it collects and uses e-mail data for advertising purposes. A “reasonable” Gmail user reading the privacy policy could not be expected to understand the e-mail scanning process.

Google says that the automated scanning of all e-mails that come through its servers — used work its spam filter but also to build user profiles and target advertisements — is vital to running its e-mail service.

But Koh rejected that argument, saying Google’s privacy policy does not mention that the site collects the content of e-mails, either between Gmail users or between Gmail users and non-Gmail users.

“Google’s alleged interceptions are neither instrumental to the provision of email services, nor are they an incidental effect of providing these services,” Koh wrote in the ruling. “The Court therefore finds that Plaintiffs have plausibly alleged that the interceptions fall outside Google’s ordinary course of business.”

“This is a very big deal,” said Electronic Privacy Information Center executive director Marc Rotenberg. “Google can no longer peer inside of everyone’s e-mail.”

Other privacy advocates said that the ruling has implications for future electronic privacy cases.

“This is a historic step for holding Internet communications subject to the same privacy laws that exist in the rest of society,” said John Simpson the privacy project director for Consumer Watchdog. “The court rightly rejected Google’s tortured logic that you have to accept intrusions of privacy if you want to send email.l The ruling means federal and state wiretap laws apply to the Internet. It’s a tremendous victory for online privacy. Companies like Google can’t simply do whatever they want with our data and emails.”

In a statement, Google defended its position. “We’re disappointed in this decision and are considering our options,” Google spokesman Matt Kallman said. “Automated scanning lets us provide Gmail users with security and spam protection, as well as great features like Priority Inbox.”

The decision came as Google celebrated the 15th anniversary of its search engine and announced that it is planning to overhaul its search product.

According to a company blog post, the firm will now allow users to ask more complex questions and use more natural language when looking up information, such as “Compare butter with olive oil.”

Google said it is also making it easier to sync notifications across devices and is revamping the look of its mobile search site.