This incident has been resolved.

The issue appeared to be resolved at 3:15pm PST, consistent with Microsoft's status page: https://status.office.com/.

Microsoft just posted a status update:
> Current status: We are currently rolling out a mitigation worldwide. Customers should begin seeing recovery at this time, and we anticipate full remediation within 60 minutes.

At approximately 1pm PST, users started reporting errors when attempting to send emails through their connected Microsoft Exchange / Office 365 accounts.

Microsoft has reported that their service is down, and they are working on bringing things back online:
1. https://status.office.com/
2. https://twitter.com/MSFT365Status/status/1371554704518352896

This issue does not affect other types of connected accounts (Gmail, IMAP/SMTP).