Yeah, based on the other responses, sounds like your case is an outlier... Were you using a custom domain, and do you know if your SPF, DKIM, DMARC were set up correctly?
I was but I was monitoring DMARC and everything seemed properly set up. It's possible that there was something I missed though. Or maybe I just happened to be on a bad IP range. Most emails to Outlook were still getting delivered but a few weren't and that was enough to be annoying.