if the 550 5.7.1 references “S3150” blocklist it can be grouped to this: Mails (MSN, Hotmail) sent from Murena blocked by Microsoft (#8380) · Issues · e / Backlog · GitLab
a newly created outlook account says it won’t accept murena mail yet, pointing to S3150 still.
this years threads on this started in early april, most are linked to the top entry. I guess once you spiked their heuristics it’s back and forth until getting a new chance.