Ask HN: How do email/SMS and SMTP servers work?

Tonight I wasted ~4 hours trying to run a self-hosted email platform locally and see if I could email myself.

I am really frustrated, but I have to admit I learned a great deal about how complex mail server operation is compared to, say, something like HTTP where you can just expose & tunnel.

It seems like a lot of the complexity stems from the fact you need multiple components for a mail system to function properly (SMTP Server, MTA/MSA, Mail Store, Delivery Agent, IMAP/POP server). And from what I gather, it seems as though ISP's can choose to block the traffic regardless, which is entirely out of your control?

Is there anyone who might be able to explain how these things work, and what the bare-minimum requirement would be for an HTTP API that could only send outbound emails?