I’d suggest you start with a simple static web server if you’re looking for a good beginner project. Use something like Nginx, and just set it up on your local network at first, then work out how to harden it, and open it up to the real internet. There’s a lot less to learn for this usage case, and it’s less likely to get you into trouble.

I say that, because, after reading through the thread, it seems you are hoping to find an (educational) use for an old computer. I did Linux and Unix admin professionally for 15 years, for some famous brands. I would NOT recommend setting up a mail server as a first project - it’s complex in ways you will never expect, and will require learning skills and knowledge that are very specific and you literally can’t “start small and build up” because a lot of the things you don’t know yet will get you into big trouble. Essentially, it’s not too hard to set up the server software, and your hardware is certainly capable of running this task, but making it safe and secure IS hard these days - especially with all the encryption and anti-spam setups you have to learn how to do.