Trying to find help for Office365 rejecting email with 550 5.7.511 Access denied, banned sender
Solution 1:
Try setting up an O365 trial account http://www.microsoft.com/en-us/microsoft-365/enterprise/e3 and try submitting a ticket there, then do your best to get through lvl1 and lvl2 nonsense and you should be able to make them escalate it internally and do something.
I would take it as a best bet although it might take more than a single ticket :(