I agree Spam is the hardest. Setting up a mailserver is pretty easy today, e.g. with NixOS-simple-mailserver you can do it with a few lines and a few minutes of config in your DNS provider, and maintenance is minimal.

But I self-host email for ~20 years, and I have not managed to get rid of Spam without not also getting false positives.

I first used SpamAssistant, in later years Rspamd, but I feel like they are just not good enough. Also I find Rspamd config pretty incomprehensible.

Hosted email like also does not solve this, e.g. GMail filters way much (e.g. important company correspondence leading to orders almost being lost because they landed in our GMail spam, so I had to turn off the spam filter entirely).