I run an email forwarding service https://hanami.run and I used to had this exact problem.

They had a dedicated page at https://sendersupport.olc.protection.outlook.com/snds/data.a... that supposed to be used for these delivery issue. I tried to submit multiple requests from there.

Even IPs that aren't sent any email in last 3 months.

Then one day the problem just disappears and I was able to send email just fine. Till this day, I still don't know what happen.

Another similar service is apple icloud, which I think even much more worse than Hotmail. At least, hotmail give you a portal that you can see some data.

With the announcement of apple icloud email service, I imagine people who are self-hosted email gonna have a lot issue with them.