When setting up DeltaChat for the use with multiple devices, the user knows exactly what to do.
Users do not think about enabling the “Send copy to self” setting (default is disabled) as they don’t know it is required in order to see sent messages on all devices.
Please rename the “Send copy to self” option to “Enable multi-device use” so the name is straightforward and easy to understand. An explanation what this option does should then be added to the manual.
Furthermore, please move this setting into the “Chats and media” section of the settings. Only a few users are having a look into the “Advanced” section for such an important feature.
I could offer to create a PR for the android implementation.