Great discussion here, not necessarily about the OP's link, but still learned a lot. Would love to contribute my 2 cents... Our app[1] sends/receives several million emails per month. Not an exaggeration, it's actually seven figures. Meaning it's more than 100k a day. Meaning it's 5-6 emails every friggin second. On average. It, of course, peaks during US daytime, up to 30 per second. We tried a looooh-ot of solution…
In the beginning I used to setup just personal "cloud", but now actually using it to send emails for other businesses I run.
In theory it should be possible to run these playbooks without all extras and just keep bare minimum to send emails (never tried it, so not sure what it will take)
Sovereign will configure postfix + dkim + spf + blacklists for you. Plus provides your imap mailboxes to receive bounces and regular mail.