First the obligatory: I'm not a lawyer or expert on GDPR. My question is if your app simply passes on messages between users of the app and doesn't store these

First the obligatory: I'm not a lawyer or expert on GDPR. My question is if your app simply passes on messages between users of the app and doesn't store these messages how can you be held accountable for what's being held on the users device, even if it is a different persons message? Your app has simply processed that users data with their consent, at the time, and passed it on as they wanted. The other issue, if the answer to the above is: 'yes', is does the GDPR expect you to be able to go into a user's phone and delete content from their phone at the request of another user? Surely that's not legal? Like I say I'm no expert, simply asking questions.