From here;

https://chaos.social/@dbrgn/114386333844571387

dbrgn@chaos.social - Here are a few interesting details about the maximally privacy-friendly protocol design:

• Everything related to synchronization between devices is completely end-to-end encrypted
• Message recipients do not know from which device a message was sent
• The Mediator Server of a device group does not know the corresponding Threema ID
• The Chat Server only sees the IP address of the Mediator Server, but not the IP address of the end devices