Not so much a question of luck, sending out sipping notifications that for example include advertising is risky. Sending a high volume of appointment reminders for the same appointment is similarly problematic.

I've never done any of that.

I don’t mean that’s the only way to trip up, there are a lot of unspoken self hosing email rules. Don’t use public data centers, don’t send news letters etc.

My email server is only used as a personal server for a few select friends and family. They absolutely do not send and have never sent anything that could remotely be considered spam. Everything in our setup is picture perfect (SPF, DKIM, DMARC, PTR records, etc). We still can't get email onto Microsoft's servers without it being marked as spam.

Interesting, marked as spam in peoples inbox is different from simply never showing up which is what happens to the vast majority of spam.