Disagree. They can do what they want with the email once it is in their queue but bouncing 'unknown' IPs is an extreme choice especially when they have SPF/dkim/dmarc/mta-sts/... all set up, and are not on any spam lists, and are hosted in a reputable data centre.

SMTP 550 means the email bounces. The sender knows but unless they're also the admin they can't do anything about it. The recipient knows nothing. In the most recent case that happened to me, it happened when I sent a reply to an @outlook I had just received a message from (and was regularly receiving emails from, but only rarely needed to reply).