Are you sure that it's MS who's blocking? Just think it's weird that they show it as delivered to recipient, sounds like they send the mail and something weird happens after.
With excessive spam scores, Microsoft silently ignores your email. Just swallows it up, reports it as delivered, but never actually does, not even in your spam folder.
It's possible that this is a technical issue or a submission server issue, but it's not uncommon for Outlook to make email disappear.
