I run a small email host, so biased a little bit. For the app-notification half, don’t put it on a mailbox host at all: smtp2go or SES, the way people are already saying here, is the right shape, because your services want an authenticated relay and nothing else, and outbound 25 is blocked on most residential lines and a lot of VPS providers anyway. The part that genuinely needs mailboxes is the friends-and-family half, and it’s worth separating: one host for the humans on your domain, one relay for the machines, so a noreply@ reputation problem never lands on the addresses people actually read. If you want FOSS on both sides, maddy is the honest answer and someone above already gave it. If you decide the human half isn’t worth running yourself, I wrote up that tradeoff at franklymail.com/vs/self-hosting, with the caveat that we’re paid and not FOSS, and we deliberately don’t do app or bulk sending, so it only answers half your question.