which makes its own mesh network, might be useful if the network is very restricted.
You can run delta chat over other networks already (emulate an email server locally on your device). One already working example of this is delta chat over yggdrasil network with a small companion app https://support.delta.chat/t/bluetooth-and-or-wi-fi-direct/3164/5.
Another example is the hermes project that uses delta chat via radio in remote regions which have no internet infrastructure. See also https://support.delta.chat/t/why-choose-delta-chat-over-xmpp/3549/4.