diff --git a/.planning/phases/17-mimecast-blast-radius-lookup/deferred-items.md b/.planning/phases/17-mimecast-blast-radius-lookup/deferred-items.md
new file mode 100644
index 0000000..f157c18
--- /dev/null
+++ b/.planning/phases/17-mimecast-blast-radius-lookup/deferred-items.md
@@ -0,0 +1,21 @@
+# Deferred Items — Phase 17, Plan 01
+
+Out-of-scope discoveries found during execution. Not fixed per SCOPE BOUNDARY
+(only auto-fix issues directly caused by the current task's changes).
+
+## Pre-existing test failures unrelated to this plan
+
+`npm test` (full suite) shows 2 pre-existing failures in
+`lib/services/analyzer/itglue-search.test.ts`:
+
+- `itglueSearch > returns capped, redacted doc snippets when the org is found`
+- `itglueSearch > tolerates per-call failures (configurations errors, flex still returns)`
+
+Both fail with a `client.getFlexibleAssetsForOrganization is not a function`
+stderr log and count-mismatch assertions. Neither `lib/services/analyzer/
+itglue-search.ts` nor its test file was touched by Phase 17 Plan 01 (which only
+modified `lib/services/mimecast-client.ts` and added
+`lib/services/mimecast-blast-radius.ts` + their test files). Last commit
+touching those files: `a0a6e7f fix(itglue): list flexible assets per type to
+satisfy API 422 requirement` — predates this plan's work. Left unfixed; not
+in scope for Phase 17.
diff --git a/lib/services/mimecast-blast-radius.test.ts b/lib/services/mimecast-blast-radius.test.ts
new file mode 100644
index 0000000..0945ad1
--- /dev/null
+++ b/lib/services/mimecast-blast-radius.test.ts
@@ -0,0 +1,227 @@
+/**
+ * mimecast-blast-radius.ts — getBlastRadius() orchestration tests.
+ *
+ * Mocks './mimecast-client' and './redis-client' entirely (following
+ * phishing-eml-service.test.ts's vi.mock() factory-mocking discipline) — no
+ * real Mimecast network calls or Redis connection are used. Synthetic inline
+ * fixtures matching MimecastDeliveredMessage/MimecastHeldMessage/
+ * MimecastThreatEvent shapes are used directly; no fixture file needed.
+ */
+
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const isMimecastConfiguredMock = vi.fn();
+const getMessageInfoMock = vi.fn();
+const searchDeliveredMessagesMock = vi.fn();
+const getHeldMessagesMock = vi.fn();
+const getThreatEventsMock = vi.fn();
+const getMimecastClientMock = vi.fn(() => ({
+  getMessageInfo: getMessageInfoMock,
+  searchDeliveredMessages: searchDeliveredMessagesMock,
+  getHeldMessages: getHeldMessagesMock,
+  getThreatEvents: getThreatEventsMock,
+}));
+
+vi.mock('./mimecast-client', () => ({
+  isMimecastConfigured: () => isMimecastConfiguredMock(),
+  getMimecastClient: () => getMimecastClientMock(),
+}));
+
+const getCachedDataMock = vi.fn();
+const setCachedDataMock = vi.fn();
+vi.mock('./redis-client', () => ({
+  getCachedData: (...args: unknown[]) => getCachedDataMock(...args),
+  setCachedData: (...args: unknown[]) => setCachedDataMock(...args),
+}));
+
+// Import AFTER the mocks are declared so vi.mock hoisting takes effect.
+import { getBlastRadius, type BlastRadiusInput } from './mimecast-blast-radius';
+
+const BASE_INPUT: BlastRadiusInput = {
+  sender: 'attacker@evil.test',
+  recipient: 'reporter@wulfconsulting.test',
+  subject: 'Urgent: verify your account',
+  dateWindow: {
+    start: new Date('2026-07-14T00:00:00.000Z'),
+    end: new Date('2026-07-15T00:00:00.000Z'),
+  },
+};
+
+describe('getBlastRadius', () => {
+  beforeEach(() => {
+    isMimecastConfiguredMock.mockReset().mockReturnValue(true);
+    getMessageInfoMock.mockReset().mockResolvedValue(null);
+    searchDeliveredMessagesMock.mockReset().mockResolvedValue({ messages: [] });
+    getHeldMessagesMock.mockReset().mockResolvedValue({ messages: [], totalCount: 0 });
+    getThreatEventsMock.mockReset().mockResolvedValue({ items: [], nextCursor: null });
+    getCachedDataMock.mockReset().mockResolvedValue(null);
+    setCachedDataMock.mockReset().mockResolvedValue(undefined);
+  });
+
+  it('returns unavailable/not_configured synchronously and never calls getMimecastClient when unconfigured', async () => {
+    isMimecastConfiguredMock.mockReturnValue(false);
+
+    const result = await getBlastRadius(BASE_INPUT);
+
+    expect(result).toEqual({ status: 'unavailable', reason: 'not_configured' });
+    expect(getMimecastClientMock).not.toHaveBeenCalled();
+    expect(searchDeliveredMessagesMock).not.toHaveBeenCalled();
+    expect(getHeldMessagesMock).not.toHaveBeenCalled();
+    expect(getThreatEventsMock).not.toHaveBeenCalled();
+  });
+
+  it('returns the cached result on a cache hit without calling any MimecastClient method', async () => {
+    const cachedResult = {
+      status: 'ok' as const,
+      matched: 1,
+      delivered: 1,
+      held: 0,
+      rejected: 0,
+      clicked: 0,
+      perRecipient: [{ recipient: BASE_INPUT.recipient, status: 'delivered' as const }],
+      source: 'fan-out' as const,
+    };
+    getCachedDataMock.mockResolvedValue(cachedResult);
+
+    const result = await getBlastRadius(BASE_INPUT);
+
+    expect(result).toEqual(cachedResult);
+    expect(searchDeliveredMessagesMock).not.toHaveBeenCalled();
+    expect(getHeldMessagesMock).not.toHaveBeenCalled();
+    expect(getThreatEventsMock).not.toHaveBeenCalled();
+    expect(setCachedDataMock).not.toHaveBeenCalled();
+  });
+
+  it('merges delivered/held/threat-event fixtures into a normalized ok result and caches it once', async () => {
+    searchDeliveredMessagesMock.mockResolvedValue({
+      messages: [
+        {
+          id: 'd1',
+          status: 'Delivered',
+          subject: BASE_INPUT.subject,
+          from: BASE_INPUT.sender,
+          fromEnv: BASE_INPUT.sender,
+          to: BASE_INPUT.recipient,
+          toDisplay: 'Reporter',
+          received: '2026-07-14T12:00:00+0000',
+          senderIP: '1.2.3.4',
+          spamScore: 0,
+          detectionLevel: 'none',
+          attachments: false,
+          route: 'inbound',
+          info: '',
+        },
+        {
+          id: 'd2',
+          status: 'Rejected',
+          subject: BASE_INPUT.subject,
+          from: BASE_INPUT.sender,
+          fromEnv: BASE_INPUT.sender,
+          to: 'other-recipient@wulfconsulting.test',
+          toDisplay: 'Other',
+          received: '2026-07-14T12:05:00+0000',
+          senderIP: '1.2.3.4',
+          spamScore: 0,
+          detectionLevel: 'none',
+          attachments: false,
+          route: 'inbound',
+          info: '',
+        },
+      ],
+    });
+    getHeldMessagesMock.mockResolvedValue({
+      messages: [
+        {
+          id: 'h1',
+          subject: BASE_INPUT.subject,
+          from: BASE_INPUT.sender,
+          fromDisplay: 'Attacker',
+          to: 'held-recipient@wulfconsulting.test',
+          toDisplay: 'Held',
+          dateReceived: '2026-07-14T13:00:00+0000',
+          reason: 'spam',
+          reasonCode: 'SPAM',
+          policyInfo: '',
+          route: 'inbound',
+          hasAttachments: false,
+          size: 100,
+        },
+      ],
+      totalCount: 1,
+    });
+    getThreatEventsMock.mockResolvedValue({
+      items: [
+        {
+          id: 't1',
+          eventType: 'click',
+          analysis: ['click'],
+          source: [],
+          direction: [],
+          status: [],
+        },
+      ],
+      nextCursor: null,
+    });
+
+    const result = await getBlastRadius(BASE_INPUT);
+
+    expect(result).toEqual({
+      status: 'ok',
+      matched: 2, // 1 non-rejected delivered + 1 held
+      delivered: 1,
+      held: 1,
+      rejected: 1,
+      clicked: 1,
+      perRecipient: expect.arrayContaining([
+        { recipient: BASE_INPUT.recipient, status: 'delivered' },
+        { recipient: 'other-recipient@wulfconsulting.test', status: 'rejected' },
+        { recipient: 'held-recipient@wulfconsulting.test', status: 'held' },
+      ]),
+      source: 'fan-out',
+    });
+    expect(setCachedDataMock).toHaveBeenCalledTimes(1);
+    expect(setCachedDataMock).toHaveBeenCalledWith(expect.any(String), result, 300);
+  });
+
+  it('clicked is 0 when no click-type analysis value is present', async () => {
+    getThreatEventsMock.mockResolvedValue({
+      items: [{ id: 't1', eventType: 'malware', analysis: ['malware'], source: [], direction: [], status: [] }],
+      nextCursor: null,
+    });
+
+    const result = await getBlastRadius(BASE_INPUT);
+
+    expect(result.status).toBe('ok');
+    if (result.status === 'ok') {
+      expect(result.clicked).toBe(0);
+    }
+  });
+
+  it('degrades to unavailable/lookup_failed (never throws) when a fan-out call rejects', async () => {
+    getHeldMessagesMock.mockRejectedValue(new Error('Mimecast API timeout'));
+
+    const result = await getBlastRadius(BASE_INPUT);
+
+    expect(result).toEqual({
+      status: 'unavailable',
+      reason: 'lookup_failed',
+      error: 'Mimecast API timeout',
+    });
+    expect(setCachedDataMock).not.toHaveBeenCalled();
+  });
+
+  it('a recipient with no delivered/held rows at all is reported as unknown', async () => {
+    const result = await getBlastRadius(BASE_INPUT);
+
+    expect(result).toEqual({
+      status: 'ok',
+      matched: 0,
+      delivered: 0,
+      held: 0,
+      rejected: 0,
+      clicked: 0,
+      perRecipient: [{ recipient: BASE_INPUT.recipient, status: 'unknown' }],
+      source: 'fan-out',
+    });
+  });
+});
diff --git a/lib/services/mimecast-blast-radius.ts b/lib/services/mimecast-blast-radius.ts
new file mode 100644
index 0000000..7be67e5
--- /dev/null
+++ b/lib/services/mimecast-blast-radius.ts
@@ -0,0 +1,195 @@
+/**
+ * Mimecast blast-radius lookup abstraction (Phase 17).
+ *
+ * Given a reported message's identity (Message-ID, sender, recipient,
+ * subject, date window), returns normalized delivery data — matched/
+ * delivered/held/rejected/clicked counts and per-recipient status — when
+ * Mimecast is configured, or a clean `status: 'unavailable'` signal (never a
+ * throw) when it isn't or when the underlying lookup fails unexpectedly.
+ *
+ * Three load-bearing facts:
+ *
+ * (a) EPHEMERAL (D-03): this lookup is a pure function call — nothing is
+ *     persisted here. If Phase 19's classifier wants to keep a result, it
+ *     writes to its own `classifications.reasons` JSONB column; that is
+ *     Phase 19's concern, not this module's.
+ *
+ * (b) `clicked` IS BEST-EFFORT (D-02): it is derived from getThreatEvents()'s
+ *     analysis[] subtype containing a click-like value. Mimecast's real
+ *     click-tracking data (TTP URL Protect logs) lives in a separate,
+ *     currently-unwrapped `/api/ttp/url/get-logs` endpoint (see
+ *     17-RESEARCH.md Pitfall 2) — this module does not call it. A value of
+ *     `clicked: 0` means "no click-type threat event found in the events
+ *     this module can see," NOT "confirmed zero clicks."
+ *
+ * (c) KNOWN LIMITATION — MULTI-TENANT GAP (D-05): this module uses only the
+ *     single global env-var-configured getMimecastClient(), NOT the
+ *     per-company `mimecast_tenants` table / getMimecastClientForTenant().
+ *     Reports belonging to companies with their own registered Mimecast
+ *     tenant (not covered by the global MIMECAST_CLIENT_ID) will return
+ *     `status: 'unavailable'` even though Mimecast is technically configured
+ *     for that company. Per-tenant resolution is a deliberate, documented v1
+ *     gap — not a silent oversight — and would be a small, mechanical
+ *     follow-up later (swap getMimecastClient() for a tenant lookup +
+ *     getMimecastClientForTenant(), same fan-out/merge logic below).
+ */
+
+import {
+  isMimecastConfigured,
+  getMimecastClient,
+  type MimecastDeliveredMessage,
+  type MimecastHeldMessage,
+} from './mimecast-client';
+import { getCachedData, setCachedData } from './redis-client';
+
+export interface BlastRadiusInput {
+  /** Optional Mimecast Message-ID — used only for supplementary body/header
+   * evidence via getMessageInfo(); it never gates or replaces the fan-out. */
+  messageId?: string;
+  /** Reported message's sender address. Required — see T-17-01: never allow
+   * a date-range-only fan-out query. */
+  sender: string;
+  /** Reporter/reported-to recipient address. Required — see T-17-01. */
+  recipient: string;
+  /** Reported message's subject. Required — see T-17-01. */
+  subject: string;
+  /** Date window to search within. Required — see T-17-01. */
+  dateWindow: { start: Date; end: Date };
+}
+
+export type BlastRadiusResult =
+  | { status: 'unavailable'; reason: 'not_configured' | 'lookup_failed'; error?: string }
+  | {
+      status: 'ok';
+      matched: number;
+      delivered: number;
+      held: number;
+      rejected: number;
+      clicked: number;
+      perRecipient: Array<{ recipient: string; status: 'delivered' | 'held' | 'rejected' | 'unknown' }>;
+      source: 'fan-out';
+    };
+
+/**
+ * Best-effort delivered/rejected split. The exact string values Mimecast
+ * returns in MimecastDeliveredMessage.status have NOT been confirmed against
+ * a live/sandbox tenant (17-RESEARCH.md A3 / Open Question #2) — only the
+ * TypeScript field name and type (`string`) are confirmed from the client
+ * code. Treat unrecognized status strings conservatively as NOT rejected
+ * (i.e. delivered) rather than hardcoding a guessed enum as if it were
+ * confirmed. Log raw values at debug level so a first real-tenant run can
+ * validate/refine this heuristic.
+ */
+function isRejectedStatus(status: string): boolean {
+  return /reject/i.test(status);
+}
+
+/** Best-effort click-type detection (D-02) — see module doc-comment (b). */
+function isClickEvent(analysis: string[] | undefined): boolean {
+  return (analysis ?? []).some((a) => /click/i.test(a));
+}
+
+export async function getBlastRadius(input: BlastRadiusInput): Promise<BlastRadiusResult> {
+  if (!isMimecastConfigured()) {
+    return { status: 'unavailable', reason: 'not_configured' };
+  }
+
+  const cacheKey = input.messageId
+    ? `mimecast:blast-radius:msgid:${input.messageId}`
+    : `mimecast:blast-radius:composite:${input.sender}:${input.subject}:${input.dateWindow.start.toISOString()}:${input.dateWindow.end.toISOString()}`;
+
+  const cached = await getCachedData<BlastRadiusResult>(cacheKey);
+  if (cached) return cached;
+
+  try {
+    const client = getMimecastClient();
+
+    // Supplementary only — body/header evidence, never gates the fan-out
+    // (17-RESEARCH.md Pitfall 1: getMessageInfo() has no status/counts).
+    if (input.messageId) {
+      await client.getMessageInfo(input.messageId);
+    }
+
+    const startStr = input.dateWindow.start.toISOString().replace(/\.\d{3}Z$/, '+0000');
+    const endStr = input.dateWindow.end.toISOString().replace(/\.\d{3}Z$/, '+0000');
+
+    // D-01 (corrected): the fan-out runs unconditionally — it is the only
+    // source of the matched/delivered/held/rejected/clicked counts.
+    const [deliveredResult, heldResult, threatsResult] = await Promise.all([
+      client.searchDeliveredMessages({
+        to: input.recipient,
+        from: input.sender,
+        subject: input.subject,
+        start: startStr,
+        end: endStr,
+      }),
+      client.getHeldMessages({ recipient: input.recipient }),
+      client.getThreatEvents(),
+    ]);
+
+    const deliveredRows: MimecastDeliveredMessage[] = deliveredResult.messages ?? [];
+    const heldRows: MimecastHeldMessage[] = heldResult.messages ?? [];
+
+    const rawStatuses = Array.from(new Set(deliveredRows.map((m) => m.status)));
+    if (rawStatuses.length > 0) {
+      console.debug('[MIMECAST-BLAST-RADIUS] raw delivered-message status values seen:', rawStatuses);
+    }
+
+    const rejectedRows = deliveredRows.filter((m) => isRejectedStatus(m.status));
+    const nonRejectedDeliveredRows = deliveredRows.filter((m) => !isRejectedStatus(m.status));
+
+    const delivered = nonRejectedDeliveredRows.length;
+    const rejected = rejectedRows.length;
+    const held = heldRows.length;
+    // matched = delivered + held; a rejected recipient was never actually
+    // delivered, so rejected is NOT included in matched.
+    const matched = delivered + held;
+
+    const clicked = (threatsResult.items ?? []).filter((t) => isClickEvent(t.analysis)).length;
+
+    // Per-recipient merge: group by the single-string `to` field. Delivered
+    // (non-rejected) rows → 'delivered', rejected rows → 'rejected', held
+    // rows → 'held' (held overwrites a same-recipient delivered entry — a
+    // held message for a recipient is the more actionable signal). Ensure
+    // the originally-queried recipient is always represented, even if
+    // neither result set returned a row for them ('unknown').
+    const perRecipientMap = new Map<string, 'delivered' | 'held' | 'rejected' | 'unknown'>();
+    for (const row of nonRejectedDeliveredRows) {
+      if (row.to) perRecipientMap.set(row.to, 'delivered');
+    }
+    for (const row of rejectedRows) {
+      if (row.to) perRecipientMap.set(row.to, 'rejected');
+    }
+    for (const row of heldRows) {
+      if (row.to) perRecipientMap.set(row.to, 'held');
+    }
+    if (!perRecipientMap.has(input.recipient)) {
+      perRecipientMap.set(input.recipient, 'unknown');
+    }
+
+    const perRecipient = Array.from(perRecipientMap.entries()).map(([recipient, status]) => ({
+      recipient,
+      status,
+    }));
+
+    const result: BlastRadiusResult = {
+      status: 'ok',
+      matched,
+      delivered,
+      held,
+      rejected,
+      clicked,
+      perRecipient,
+      source: 'fan-out',
+    };
+
+    await setCachedData(cacheKey, result, 300);
+    return result;
+  } catch (err) {
+    // T-17-02: log err.message ONLY, never full Mimecast response bodies
+    // (which may contain other recipients' subjects/content).
+    const message = err instanceof Error ? err.message : String(err);
+    console.error('[MIMECAST-BLAST-RADIUS] lookup failed', message);
+    return { status: 'unavailable', reason: 'lookup_failed', error: message };
+  }
+}