I ran my own mail server from 1998 through 2019, and set up a FreeBSD mail server as one of my first contract jobs in 1998 or 1999. I used Sendmail, Exim, Postfix, and qmail at various times. I switched to mail-in-a-box in 2014, and contributed a few minor fixes, then (which I'd forgotten about until I idly looked to see, just now).

Throughout 20 years of running my own mail server for companies, friends, and myself, the additional effort to get commercially-run mail servers to accept mail was both annoying and random ("oh, look, hosted Outlook has started rejecting our mail again..."), and sometimes they don't even send a standard response but just "accept" and blackhole the email. Eventually you find out that someone else in the /24 you're in at Rackspace or DigitalOcean is happily running an open relay, and that's why your IP is having problems. Or any of a dozen similar things.

In 2019, having gotten very tired of this, I gave up and moved my mail handling to Amazon Workmail and SMS, and after setting it up properly once, it's been trouble-free and maintenance-free for half a decade. Compared to some solutions, it's expensive, but not in absolute terms.