My SMTP mail server is relatively new, and it took a few weeks, but Microsoft has blocked it’s IP address. The non-delivery report (NDR) says this:
For further assistance, please send mail to postmaster.
Care to guess what gets rejected because I’m on their block list? That’s right: email@example.com and firstname.lastname@example.org
For what it is worth, I’m currently getting a 9.8 out of 10 on mail-tester.com: https://www.mail-tester.com/test-1dcmfmxh8