According to my (very non-expert) investigations with mxtoolbox.com, disroot.org appears to be on a backscatterer blacklist.
Listing on Backscatterer blacklist indicates that your server is issuing “backscatter” in the form of Non-Delivery Reports (bounces) to external users, or misdirected autoresponders and sender callouts. Much of this is a function of how your server configuration is set to handle emails directed to users that do not exist on the server, and can cause many problems especially if you are hit with a directory-harvesting attack. Simply requesting delisting and paying their fee will do nothing to help you correct the problem and will give you no guarantee that you won’t get blacklisted again.
There is conflicting advice about what to do about this, including Do Nothing and the listing will timeout and the problem will go away. More detail from mxtoolbox here.
Alternatively, you can fill out this form from Microsoft and they will unblock your IP address manually.
I hope this is helpful, @muppeth and apologise if it’s just distracting bullsh*t.