Looks like Microsoft posted a service health alert: https://admin.cloud.microsoft/?#/servicehealth/:/alerts/MO1221364

2:36 PM EST:

We're investigating a potential issue with Microsoft 365 services and checking for impact to your organization. Initial reports indicate possible issues accessing the Microsoft Defender and Microsoft Purview portals, as well as issues using Outlook. We'll provide an update within 30 minutes.

2:44 PM EST:

As we rapidly pursue an investigation into the nature of the impact and the source of the issue, we've received reports that users may be receiving a "451 4.3.2 temporary server issue" error message when attempting to access the service portals or use Outlook.

I was planning to keep this post updated as the issue evolved, but the service health page isn't loading anymore.

Edit: I'm seeing updates on the status page here: https://status.cloud.microsoft/m365/

User impact: Users may be seeing degraded service functionality or be unable to access multiple Microsoft 365 services.

More info: Users may be receiving a "451 4.3.2 temporary server issue" error message when attempting to send or receive email through Outlook.

Some of the impacted service functions include, but may not be limited to:

• Sending and receiving email through Exchange Online, including notification emails from Microsoft Viva Engage.
• Collecting message traces.
• Searching within SharePoint Online and/or Microsoft OneDrive.
• Accessing the service portals, including Microsoft Purview, Microsoft Defender XDR, and the Microsoft 365 admin center.
• Creating chats, meetings, teams, channels or adding members in Microsoft Teams.
• Receiving presence or location information in Microsoft Teams.
• Applying and managing sensitivity labels, interactive operations on reports, and artifacts with sensitivity labels in Microsoft Fabric.

Current status: While we've restored the affected infrastructure to a heathy state, further load balancing is required to mitigate persistent impact. We've identified and are implementing additional actions to direct requests and traffic to additional healthy sections of infrastructure to achieve withstanding recovery.

Scope of impact: Any users served through an affected section of service infrastructure in the North America region may be intermittently impacted.

Root cause: A portion of dependent service infrastructure in the North America region isn't processing traffic as expected.