Nothing about the program itself is subscription based. All of the normal features of an email client (that you would also find in Thunderbird) are available for free. You only need to pay if you want to use their services like Send later, read receipts or link tracking, because these requires backend servers and actually costs the money.
They have an install script that makes this much easier. https://github.com/tailscale-dev/deck-tailscale
I summed up the steps:
Create an account at https://login.tailscale.com/start
Open Konsole and copy-paste the following commands, then hit enter to run them:
git clone https://github.com/tailscale-dev/deck-tailscale.git; cd deck-tailscale
sudo bash tailscale.sh
source /etc/profile.d/tailscale.sh
sudo tailscale up --qr --operator=deck --ssh
This will give you a QR code, that you need to scan with your phone. You will have to log in to Tailscale to add the Steam Deck to your Tailscale network.Try running
sudo tailscale update
If this works, i.e. if you don’t get any error messages, run
sudo tailscale set --auto-update
If you use Decky Loader, I recommend installing the Tailscale Control plugin, which lets you control Tailscale from the Steam menu. You can also use KTailctl to control it from desktop mode.