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It could be server side spam filtering (not all spam messages make it to your spam folder with any provider - the worst offenders are often just thrown out). Just saying it is possible that this is an over-aggressive spam filter vs. Apple taking such an invasive measure. Although, Apple has done similar crazy things before, so who knows.
That is the wrong way to run a mail server. I know how common it is at big providers, but it's still wrong. You should REJECT a message if you won't deliver it. If it was a legit message inappropriately REJECTed, then the server that's relaying it can generate a bounce back to the sender, and something can be figured out. Dropping a message on the floor like that, after you have promised to deliver it is almost alway…
Whether or not that's appropriate is another thing, but that is probably the rationale behind it.