SharePoint External Sharing Not Working: A Read-First Org, Site, Domain, and Entra B2B Triage

SharePoint administrators face challenges with the Everyone except external users claim and external sharing policies. EEEU is a special SharePoint claim, not a standard Microsoft 365 security group, requiring a functional chain to operate. Similarly, external sharing is governed by a stack of settings where the most restrictive layer prevails. Issues often arise from organization-level, site-level, or domain-level restrictions, meaning simple switches cannot resolve complex access denied errors or People Picker failures.