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.
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.