Microsoft is blocking our IP address from sending emails to our own customers

Microsoft has blocked our IP address and refuses to tell us what the issue is, so we cannot send any emails to our customers using Outlook, Hotmail, etc. Our report to the Better Business Bureau also didn't help. They pointed us to a list of steps that don't work and are nonsensical with improper English. When trying to delist we simply get a message stating that the IP is not available for mitigation. The only step left is to file a civil case, which we are trying to avoid. Anyone else have this issue? How do we reach someone in corporate?