Looks like Outlook may be having issues. I was fumbling around trying to get 2FA codes from one of our providers, and discovered no emails were arriving. Microsoft's status page is still showing all green, but we're down, and it seems other people are too.

Getting SMTP errors to outlook emails: 451 4.3.2 Temporary server error. Please try again later ATTR2

Finally an hour later, Microsoft acknowledged the issue: https://status.cloud.microsoft/m365/referrer=serviceStatusRe...