Apple Inc. said it has rejected an appeal from the email app Hey, saying the software violated App Store rules related to in-app purchases.

“The HEY Email app is marketed as an email app on the App Store, but when users download your app, it does not work,” Apple said in a letter to the developer Basecamp that was obtained by Bloomberg News. “Users cannot use the app to access email or perform any useful function until after they go to the Basecamp website for Hey Email and purchase a license to use the HEY Email app.”

Apple also said in the letter that Basecamp apps have “not contributed any revenue to the App Store over the last eight years.”

Earlier this week, a Basecamp executive complained on Twitter about Apple’s App Store rules and said this was an antitrust issue.