What Happened?

TL;DR: Microsoft blocked our server’s IP address, we’re unable to send confirmation mails to Outlook mailboxes.

We’ve noticed that our outgoing mail server is unable to deliver mail to Outlook mail servers over the past few months.
Mails from our IP address are simply rejected without any useful explanation.
We’ve contacted our service provider and they tried to contact Microsoft support to get the restriction lifted.
That didn’t give any result. So we tried to contact them (Microsoft) ourselves, they were uncooperative again.
In conclusion, if you want to use our services that require email confirmation, you’ll have to avoid Outlook.
Sorry for the inconvenience.