I got lost with setting up a nice inbox downloader to store all my emails on a HDD attached to my RPI4, but haven't quite mastered the SMTP server part or found the right software to run on it. It's currently powered off waiting for a reflash of the SD Card so I can try again. The end goal for mine is to set up fetchmail and have it grab from my inboxes then imap capabilities so I can read it in Thunderbird. (Don't talk to me about webmail, I know it's the way but I'm older than Star Wars (Original one) and am stuck in my ways. Now get off of my lawn!
Seriously though, I have tinkered with it before as an AdguardHome Server, but somehow, my latency increased so I dropped that. Most of it's life was spent hosting Home Assistant on it until I moved that to the umm...more controversial Proxmox VM method. I'm also on the fence about setting up the Raspberry Pi Nextcloud on it. (Maybe).
Here is a good resource for 36 different things you could possibly do with yours.