Apple has a "secret list" too it seems, I had one case of this with one domain. When I called I explained to normal tech support the issue, they had me escalated where I explained the issue in a bit more detail. Within an hour or two I had a call back from Apple support telling me that the domain had been removed, I didn't pry any more I just figured since they have the right to deny email for whatever reason then have the right to do this. This came after looking over logs, and some packet captures, to make sure it was being delivered to their servers before making the call to Apple. Nothing indicated any type of failure/deferred/blocked from looking at those logs/captures.