delivery_target_unreachable· reason_code· applies to event_envelope, service_callSub-reason carried by the ak.invite.revoke that moves a pending Invite to target_state=send_failed: the delivery service could not reach the private invite delivery target after its retry budget. It is a delivery diagnostic only and MUST NOT leak the 3PID plaintext, the invite token or a verification code. See zh/models/governance-objects.md section 5 and zh/sync/third-party-invites.md section 6.1.