First idea: Message deleting on demand
At the Moment on chatmail servers you can choose delete message automatically or after download. Especially if you receive many messages then your inbox gets very quickly full, so I asked me if it would be possible to implement something like delete messages on demand so if you inbox is like 80% it deletes older messages to make some space on the server even if they are not that old.
Second idea: Multi-device delete on download
As alternative I thought about something like delete on download (multi-device), so instead of deleting the incoming mails directly it could wait some 15 seconds or something like this that all devices which are online could follow the chat and download the incoming message before it get deleted.