Unable to invite external B2B guest users – invitations blocked due to suspicious activity

Hello Microsoft Team,

We are unable to invite any external B2B guest users to our Microsoft Entra ID tenant.

All invitation methods fail (Entra Admin Center, SharePoint Online, OneDrive and Microsoft Graph API) with the following error: "Invitations are blocked for this directory due to suspicious activity. Please contact Microsoft support for help."

A Microsoft Support investigation has already been completed under case 2608061420000987.

The investigation confirmed that:

• Microsoft Graph endpoint POST /v1.0/invitations returns 403 Forbidden.
• The issue is not related to SharePoint, OneDrive or tenant configuration.
• The restriction appears to be enforced by Microsoft Entra ID service-side protection mechanisms.

Unfortunately, the standard support forms do not provide an option to report a service-side tenant block, and the issue cannot be resolved through tenant configuration changes.

Could a Microsoft moderator please review this tenant and advise how the Entra ID restriction can be removed?

I can provide the Tenant ID and any additional information via private message.

Thank you for your assistance.

Kind regards