SharePoint and OneDrive Site User ID Mismatch Explored

SharePoint and OneDrive can experience "Access Denied" errors due to Site ID mismatches, even if user's appear healthy. SharePoint uses a unique identifier (PUID) and User Information List (UIL) to grant access, separate from the UPN. Issues arise when a new user gets the same UPN as a previous employee, but inherits no previous access. Terminated users remain in SharePoint's UIL for historical reasons, without posing a security risk. Preventing mismatches involves avoiding UPN reuse, restoring accounts when rehiring within a timeframe, and renaming instead of recreating UPNs. During tenant/domain changes, handle identity mapping with migration tools. UPN alteration before deletion, and documenting existing user access, aids in conflict resolution. Cleanup involves removing the old user from relevant SharePoint sites or inventorying for re-permissioning. Resolving mismatches requires cleaning up the old user or identifying sites for new account access.