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.
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.
If the recipient is using a personal microsoft / outlook / hotmail account the email gets delivered with a spam score of 9.
If the recipient is a MS 365 account, blackhole it is.
We even set up our own MS 365 to prove this.