It is not that simple to run your own email server anymore. Big providers like Google will treat emails from your server as spam and you will have a difficult time having the mail properly delivered. So big tech has effectively squeezed out federated email.

Leaves room for privacy respecting platforms.

Distrobox is pretty easy to setup. Then you can install what you need without touching the SteamOS install.

https://www.gamingonlinux.com/2022/09/distrobox-can-open-up-the-steam-deck-to-a-whole-new-world/