They do maintain their own blacklist, which doesn't seem to be listed on any of the public monitors. When I had this issue it did give a helpful bounce message:

They do maintain their own blacklist, which doesn't seem to be listed on any of the public monitors. When I had this issue it did give a helpful bounce message: Client host blocked using FBLW15; To request removal from this list please forward this message to delist@messaging.microsoft.com (in reply to RCPT TO command) I did as it said and forwarded the message to that address, and got an automated reply and my IP removed a few days later. Maybe your situation is different since you're not getting a bounce message, but try emailing that address if you haven't already.