Using classic Outlook for Windows in business environments
I understand how frustrating this must be, especially when you rely on email to run your business.
Based on your description, this issue appears to be related to Microsoft's ongoing service incident (Incident ID: MO1550821), which is affecting connectivity to multiple Microsoft 365 services, including Exchange Online, Microsoft Teams, and SharePoint Online.
If you are an admin, you can review Microsoft's service health advisory directly through the Microsoft 365 Admin Center. If you're an end user without admin permissions, kindly reach out to your Microsoft 365 administrator for assistance. As Microsoft has not yet confirmed a full restoration time, please continue to follow the incident updates in the Service Health dashboard, where Microsoft will share further information as recovery progresses.