There are a few standards now, DMIM, 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.