First based on everything I have ever heard, don’t host your own email server. If you want to learn it that’s fine, but don’t do it for something you actually rely on. My understanding it’s a constant miserable slog to keep it functioning.
Second for getting started, get a cheap used computer and install Linux on it. Also be prepared to wipe it and install something different. Docker may be a good place to go next. You can find lots of guides online, there are tones of apps setup to run with docker you can test out. You could also explore virtual machines. Both have the advantage of making it easier to experiment with different things.
Proxmox is basically a VM management software that runs in a Debian install. You may be able to add it to your current install, but I am not certain about that.