I went through this hell last year when trying to tell my customers I had to change payment processors.

In my case it turns out I was victim of Google having a beef with afraid.org years ago (the DNS authoritative record for my domain). 100% of my emails to gmail were going to spam, as soon as I switched NS record to fastmail (with same zone file) I hit the inbox.

I never would have caught it without using this site: https://www.gmass.co/inbox

I did not show up on any of the major blacklists with mxtoolbox, this took months to figure out and I felt like I was going insane with hardly anyone responding to my emails.