As many as 100 mobile application developers may be getting bad news this week in their mailboxes: According to California online privacy policies, their apps violate the law.

The California Online Privacy Act requires any online commercial service with customers residing in California to post a privacy policy that's visible to those customers. 

The law's offenders will be given 30 days to post a privacy policy that states what personal information is being gathered from users and how that information is being used, according to a statement by California Attorney General Kamala Harris.

“We have worked hard to ensure that app developers are aware of their legal obligations to respect the privacy of Californians, but it is critical that we take all necessary steps to enforce California’s privacy laws,” Harris said in the release.

So far, 20 letters have been sent out and may take up to two to three weeks for all letters to be delivered, Shum Preston, a spokesman for California Department of Justice told Inc.

Making sure that a privacy policy is readily visible to consumers should not be a massive undertaking, according to Tyler Newby, an attorney at Fenwick & West, a law firm that advises tech start-ups.

"The process of making a privacy policy available on the App Store shouldn't be a big issue for a smaller companies if they already have a privacy policy in place as most do," Newby said, adding that none of the law firm's clients have received a letter yet.

"Where there may be more difficulty for compliance is if the attorney general's office or regulators start making requirements on what language is adequate or not adequate within the privacy policy," he said.

Letters will be sent first to companies with the most popular mobile apps in violation of the law. Delta Airlines, United Continental Holdings, and online restaurant reservation service OpenTable are among the companies that have received the letter, Bloomberg News previously reported.

Companies can be fined up to $2,500 for each download of a non-compliant app under the state privacy law.