I think your question could be generalized further:
Could chat participants exchange messages in ways other than through their respective chatmail server?
Iroh and the recently added support for federation delivery over HTTPS shows that email is just one of the possible transports. As long as batches of messages are properly framed, (compressed), signed and encrypted, it shouldn’t matter how it reaches the recipient:
- Bluetooth piconet
- Wifi P2P Direct
- sneakernet over a data mule: i.e., the device of mutual contacts who you regularly meet in person could sync automatically
- LAN, including through switches and wifi AP
- through a chatmail server, usually through WAN
Briar already supports such a hybrid delivery scheme over Bluetooth and Tor: