Delivery to Microsoft's products has been patchy for several years. I am not remotely an email deliverability expert but I have made different SMTP service choices based on how companies approach shared-IP sending plans.

Some can affordably deliver to Microsoft because they aggressively police what they are actually sending on their customers' behalf; others don't get involved and you end up having to spend a fortune on a dedicated IP.

The DMARC thing is just another part of this; one of my clients discovered this silent failure themselves a few weeks back and addressed this.