Many of these systems keep redoing the same work over and over which seems wasteful.
What I'd like is a "mail reverse proxy" that does all the work to manage DNS, SPF, DKIM, DMARC, etc and handles sending and receiving emails, but doesn't do any storage or user management. Instead it forwards mail from/to the "real" mailserver sitting somewhere in a private network. (Maybe using LMTP [2]?)
This way you could roll the dice until you get a $5 VPS with a clean static IP and just park it there permanently, where it does nothing but sends and receives emails from your real mail server wherever you want to host it. Kinda like a PO box. You never have to worry about upgrading it to get more storage, or switching providers and losing your IP, if it gets hacked the worst it can do is spy on live email traffic and send spam until its patched.
Why doesn't this exist already?