I mean, if you read the full tweet thread, it says they broke rule 3.1.1, and 3.1.3(b) > Apps that operate across multiple platforms may allow users to access

I mean, if you read the full tweet thread, it says they broke rule 3.1.1, and 3.1.3(b) > Apps that operate across multiple platforms may allow users to access content, subscriptions, or features they have acquired in your app on other platforms or your web site, including consumable items in multiplatform games, provided those items are also available as in-app purchases within the app. Basically Apple says if you're offering a multi-platform service - which this e-mail client clearly is - then you must provide the option to buy the service in the App using apple's in-app purchase system.