If you want to experiment with Teredo today, you need a Windows machine (Linux is possible but harder; macOS dropped support).
You can't mail the letter directly.
If it says "Media Disconnected," you can try to wake it up: teredo
Because this info is embedded in the address, anyone on the internet who knows your IPv6 address knows exactly which IPv4 address and UDP port to send the packet to. If you want to experiment with Teredo today,