If you're a ProtonMail user, you could run the Proton Bridge on a Linux system in your network, then use PuTTY on Win XP to forward the ports from there to Wind

If you're a ProtonMail user, you could run the Proton Bridge on a Linux system in your network, then use PuTTY on Win XP to forward the ports from there to Windows. Then Thunderbird is only sending bridge-specific passwords unencrypted to localhost, and it's all modern encryption from there on out.