diff --git a/lib/services/mimecast-blast-radius.test.ts b/lib/services/mimecast-blast-radius.test.ts
index 0945ad1..120bb88 100644
--- a/lib/services/mimecast-blast-radius.test.ts
+++ b/lib/services/mimecast-blast-radius.test.ts
@@ -224,4 +224,62 @@ describe('getBlastRadius', () => {
       source: 'fan-out',
     });
   });
+
+  it('uses an injected tenant client via options.client and never calls getMimecastClient', async () => {
+    const tenantGetMessageInfoMock = vi.fn().mockResolvedValue(null);
+    const tenantSearchDeliveredMessagesMock = vi.fn().mockResolvedValue({ messages: [] });
+    const tenantGetHeldMessagesMock = vi.fn().mockResolvedValue({ messages: [], totalCount: 0 });
+    const tenantGetThreatEventsMock = vi.fn().mockResolvedValue({ items: [], nextCursor: null });
+    const fakeTenantClient = {
+      getMessageInfo: tenantGetMessageInfoMock,
+      searchDeliveredMessages: tenantSearchDeliveredMessagesMock,
+      getHeldMessages: tenantGetHeldMessagesMock,
+      getThreatEvents: tenantGetThreatEventsMock,
+    };
+
+    const result = await getBlastRadius(BASE_INPUT, { client: fakeTenantClient as any, cacheScope: 'company-123' });
+
+    expect(result.status).toBe('ok');
+    expect(getMimecastClientMock).not.toHaveBeenCalled();
+    expect(tenantSearchDeliveredMessagesMock).toHaveBeenCalledTimes(1);
+    expect(tenantGetHeldMessagesMock).toHaveBeenCalledTimes(1);
+    expect(tenantGetThreatEventsMock).toHaveBeenCalledTimes(1);
+    // Global client's mocked methods must not have been touched.
+    expect(searchDeliveredMessagesMock).not.toHaveBeenCalled();
+  });
+
+  it('runs the fan-out with an injected tenant client even when isMimecastConfigured() is false', async () => {
+    isMimecastConfiguredMock.mockReturnValue(false);
+    const tenantSearchDeliveredMessagesMock = vi.fn().mockResolvedValue({ messages: [] });
+    const tenantGetHeldMessagesMock = vi.fn().mockResolvedValue({ messages: [], totalCount: 0 });
+    const tenantGetThreatEventsMock = vi.fn().mockResolvedValue({ items: [], nextCursor: null });
+    const fakeTenantClient = {
+      getMessageInfo: vi.fn().mockResolvedValue(null),
+      searchDeliveredMessages: tenantSearchDeliveredMessagesMock,
+      getHeldMessages: tenantGetHeldMessagesMock,
+      getThreatEvents: tenantGetThreatEventsMock,
+    };
+
+    const result = await getBlastRadius(BASE_INPUT, { client: fakeTenantClient as any });
+
+    expect(result.status).toBe('ok');
+    expect(getMimecastClientMock).not.toHaveBeenCalled();
+    expect(tenantSearchDeliveredMessagesMock).toHaveBeenCalledTimes(1);
+  });
+
+  it('degrades to unavailable/lookup_failed when searchDeliveredMessages swallows an error internally (Bug 1 defense-in-depth)', async () => {
+    searchDeliveredMessagesMock.mockResolvedValue({
+      messages: [],
+      error: 'err_track_and_trace_invalid_end_date',
+    });
+
+    const result = await getBlastRadius(BASE_INPUT);
+
+    expect(result).toEqual({
+      status: 'unavailable',
+      reason: 'lookup_failed',
+      error: 'err_track_and_trace_invalid_end_date',
+    });
+    expect(setCachedDataMock).not.toHaveBeenCalled();
+  });
 });