The problem lies with the big e-mail providers marking your e-mail as spam, even if you have everything configured right (MX, SPF, DKIM, DMARC, etc.) and you're not on any blacklists. They use their mysterious opaque algo to determine that your e-mail is spam and you have no idea what you could do to not get in peoples junk boxes.
And that’s not hard?
Running a modern mail server, a spam filter, a self-hosted webmail, and learning just enough about DNS and networking to get mail through: this is what’s not that hard.
Figuring out how to stay whitelisted, or accept ending up in people’s filters, asking them to whitelist you while your public IP gradually gains trust, is a job.
I just wish these big parties would give you a more formal way to make clear you're not a spammer.
There is only one metric of an honest mailserver and that is time spent not blacklisted.
This is true "proof of work" on behalf of the mail server admin.
Running a mailserver on an IP range easily accessible to spammers (e.g. a cloud provider) gives you a negative start. You need to invest in "good IP addresses". I will gladly pay money to anyone who wants to do this. There are medium-sized players who deliver affordable, top-notch service. Turns out $5/mo. is enough for human service, because something that just works doesn't need a lot of it.