

If you are going to run Jellyfin or some other media sharing, the key is if you need to transcode media (recompress because the playback device cannot handle it or not). Likely not, nowadays, but research that. If you need transcoding, research; you might get by with an old CPU, or maybe hardware transcoding support, but it’s difficult.
Outside transcoding, for file sharing/streaming, every simultaneous client will require additional horsepower and disk transfer usage. If you are the sole client, then likely you can do with an old CPU. But if you and three people more in your household are going to be using the system at the same time, it might be a bit complex.
One of my home servers is a 4gb of RAM, with a “Intel® Celeron® CPU G1610T @ 2.30GHz”. It’s very old and low end, but for file sharing it works quite well, but it rarely has more than a single simultaneous user.
Reminder that you can go for hybrid approaches; receive email and host IMAP/webmail yourself, and send emails through someone like AWS. I am not saying you can’t do SMTP yourself, but if you want to just dip your toes, it’s an option.
You get many of the advantages; you control your email addresses, you store all of the email and control backups, etc.
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And another thing: you could also play with https://chatmail.at/relays ; which is pretty cool. I had read about Delta Chat, but decided to play with it recently and… it’s blown my mind.