There are a few standards now, DKIM, SFP, DMARC, maybe more now, I don’t know. If you send emails without these configured correctly the reputation of the domain and IP are lowered.

Past some internal threshold, you go from inbox to spam, and from spam to silently dropped.

Further, if you send too many emails in a short time, or more emails than usual, your reputation is lowered.

I’m sure there’s more, but these are the kind of things that make it difficult. You make a config error, don’t realize, then people start not getting your emails. You fix the config, but there’s no way to get the reputation back and nobody at Microsoft or Google to ask to re-evaluate you.

Deliverability to major providers like Google or Microsoft. Can be just getting your emails flagged as spam, or them being sikently dropped and never delivered even to spam. Making it impossible know if your emails are being ignored by the recipient or not even delivered to their inbox. It’s also impossible to troubleshoot.

Mostly reputation of your IP address and domain, things which are hard to untangle. If you manage to get a clean IP you might be all clear.

There’s other configurations that are required and if not right can harm your reputation, it isn’t something you can set and forget.

I hate that it’s come to this, but you are right.

It’s not that it’s too difficult, it’s that there are too many things beyond your control due to the central duopoly of Google and Microsoft for email. If you end up in their bad graces it’s hard to get out, and they don’t care about you, there’s no support or someone to talk to to get off the ban list.