If you start a new mail server in 2022 (or migrate from a previous address), you have to apply to be whitelisted by outlook.com and the many domains owned by Microsoft. They no longer accept mail from servers they haven't seen before. Companies such as google, microsoft, and facebook would rather that email died, and are actively working to destroy it through neglect, so that people will shift their messaging to prop…
You can't make this up. They pass all abuse reports throuh their spam filter and this affects your server's reputation. Then at some point they start rejecting emails from you with a 5xx code that stands for "your IP is on our blacklist". Contacting "Hotmail sender support" will yield "We see no problems on our end, kindly fuck off". You have to then go through a ritual of summoning a manager, taking anywhere from 2 to 5 days, at which point they will say "we put a mitigation in place, will take up to 48 hours to propagate", but it works right away. If it doesn't, it means that they mistyped your server address and the next chance to ask them to fix this will be, you guessed it, in 48 hours. So a week of bounces in total.
Ran into this 3 times in a course of a year, then took a closer look at the logs and - yep - in all three cases it was triggered by sending a report to abuse@outlook.com.
It's just such a magnifecent clusterfuck it's jaw-dropping.