diff --git a/src/features/hardening/LegacyProtocolsPage.tsx b/src/features/hardening/LegacyProtocolsPage.tsx
index 7b3add4..0ada5a3 100644
--- a/src/features/hardening/LegacyProtocolsPage.tsx
+++ b/src/features/hardening/LegacyProtocolsPage.tsx
@@ -25,8 +25,10 @@ import { useAccountStore } from '@/stores/accountStore';
 import { toast } from '@/hooks/use-toast';
 import { cn } from '@/lib/utils';
 import {
+  ago,
   CONFIRM_PHRASE,
   describeListener,
+  impactEntries,
   fetchProtocolPolicy,
   phraseMatches,
   PolicyUnavailable,
@@ -35,6 +37,7 @@ import {
   type PolicyListener,
   type ProtocolPolicy,
   type ProtocolRow,
+  type RecentUse,
 } from './protocolPolicy';
 
 type Load = { kind: 'loading' } | { kind: 'ready'; policy: ProtocolPolicy } | { kind: 'error'; message: string };
@@ -156,6 +159,8 @@ export function LegacyProtocolsPage() {
 
       <ProtocolTable rows={protocolRows(policy, listeners)} off={off} />
 
+      {!off && policy.recentLegacyUse && <ImpactPanel recent={policy.recentLegacyUse} />}
+
       {(off || confirming) && <Statement listeners={listeners} />}
 
       {!off && canUpdate && !confirming && (
@@ -316,6 +321,48 @@ function ProtocolTable({ rows, off }: { rows: ProtocolRow[]; off: boolean }) {
   );
 }
 
+/**
+ * The impact panel (LP-15): who would notice, shown before anything can
+ * change. With nobody, it says so in one line.
+ */
+function ImpactPanel({ recent }: { recent: RecentUse[] }) {
+  const { t, i18n } = useTranslation();
+  const entries = impactEntries(recent);
+  // Read once, when the panel appears: "2 days ago" needn't tick.
+  const [now] = useState(() => Date.now());
+  if (entries.length === 0) {
+    return (
+      <p className="rounded-xl border px-4 py-3 text-sm text-muted-foreground">
+        {t('legacyProtocols.impactNone', 'No account used a legacy mail app in the last 30 days.')}
+      </p>
+    );
+  }
+  return (
+    <section className="space-y-2 rounded-xl border p-4 text-sm">
+      <p>
+        <strong>
+          {t('legacyProtocols.impactCount', {
+            count: entries.length,
+            defaultValue_one: '1 account used a legacy mail app in the last 30 days.',
+            defaultValue_other: '{{count}} accounts used a legacy mail app in the last 30 days.',
+          })}
+        </strong>{' '}
+        {t('legacyProtocols.impactLead', 'Their mail apps will stop working the moment you turn this on:')}
+      </p>
+      <ul className="max-h-72 space-y-1 overflow-y-auto">
+        {entries.map((entry) => (
+          <li key={entry.name} className="flex flex-wrap gap-x-2">
+            <span className="font-medium">{entry.name}</span>
+            <span className="text-muted-foreground">
+              {entry.protocols.join(', ')} · {ago(entry.lastUsedAt, now, i18n.language)}
+            </span>
+          </li>
+        ))}
+      </ul>
+    </section>
+  );
+}
+
 /** The statement (LP-16), at server scope, with the firewall note (LP-20). */
 function Statement({ listeners }: { listeners: PolicyListener[] }) {
   const { t } = useTranslation();
diff --git a/src/features/hardening/protocolPolicy.test.ts b/src/features/hardening/protocolPolicy.test.ts
index 46ecfc0..6bd822f 100644
--- a/src/features/hardening/protocolPolicy.test.ts
+++ b/src/features/hardening/protocolPolicy.test.ts
@@ -5,7 +5,15 @@
  */
 
 import { describe, expect, it } from 'vitest';
-import { CONFIRM_PHRASE, parsePolicy, phraseMatches, protocolRows, type ProtocolPolicy } from './protocolPolicy';
+import {
+  ago,
+  CONFIRM_PHRASE,
+  impactEntries,
+  parsePolicy,
+  phraseMatches,
+  protocolRows,
+  type ProtocolPolicy,
+} from './protocolPolicy';
 
 // As inbuxa:ProtocolPolicy/get sends it: listeners keyed by the policy's own property names.
 const WIRE = {
@@ -82,3 +90,33 @@ describe('phraseMatches (LP-17)', () => {
     expect(phraseMatches('')).toBe(false);
   });
 });
+
+describe('the impact panel (LP-15)', () => {
+  it('reads nothing from a server too old to say, and an empty list as nobody', () => {
+    expect(parsePolicy(WIRE).recentLegacyUse).toBeNull();
+    expect(parsePolicy({ ...WIRE, recentLegacyUse: [] }).recentLegacyUse).toEqual([]);
+  });
+
+  it('shows each account once, with every protocol it used and its latest use', () => {
+    const recent = parsePolicy({
+      ...WIRE,
+      recentLegacyUse: [
+        { accountId: 'a', name: 'maria@example.org', protocol: 'submission', lastUsedAt: 100 },
+        { accountId: 'a', name: 'maria@example.org', protocol: 'imap', lastUsedAt: 300 },
+        { accountId: 'b', name: 'ada@example.org', protocol: 'pop3', lastUsedAt: 200 },
+        { accountId: 'c', name: 'bad' },
+      ],
+    }).recentLegacyUse!;
+    expect(impactEntries(recent)).toEqual([
+      { name: 'maria@example.org', protocols: ['IMAP', 'SMTP submission'], lastUsedAt: 300 },
+      { name: 'ada@example.org', protocols: ['POP3'], lastUsedAt: 200 },
+    ]);
+  });
+
+  it('says how long ago in words', () => {
+    const now = Date.UTC(2026, 8, 21);
+    expect(ago(now - 2 * 86400_000, now, 'en')).toBe('2 days ago');
+    expect(ago(now - 3 * 3600_000, now, 'en')).toBe('3 hours ago');
+    expect(ago(now - 10_000, now, 'en')).toBe('this minute');
+  });
+});
diff --git a/src/features/hardening/protocolPolicy.ts b/src/features/hardening/protocolPolicy.ts
index 7f76e19..126f701 100644
--- a/src/features/hardening/protocolPolicy.ts
+++ b/src/features/hardening/protocolPolicy.ts
@@ -38,6 +38,27 @@ export interface ProtocolPolicy {
   lockedProtocols: string[];
   /** What turning the switch off would close, whichever way it is set now (LP-16). */
   wouldClose: PolicyListener[];
+  /**
+   * Who signed in over a legacy protocol in the last 30 days (LP-15), or null
+   * from a server too old to say -- which is not the same as nobody.
+   */
+  recentLegacyUse: RecentUse[] | null;
+}
+
+/** One account's last sign-in over one legacy protocol, as the server reports it. */
+export interface RecentUse {
+  accountId: string;
+  name: string;
+  protocol: string;
+  /** Milliseconds since the epoch. */
+  lastUsedAt: number;
+}
+
+/** One account on the impact panel: every protocol it used, and when it last did. */
+export interface ImpactEntry {
+  name: string;
+  protocols: string[];
+  lastUsedAt: number;
 }
 
 /**
@@ -71,14 +92,75 @@ export function parsePolicy(raw: Record<string, unknown>): ProtocolPolicy {
       ? raw.lockedProtocols.filter((p): p is string => typeof p === 'string')
       : [],
     wouldClose: parseListeners(raw.wouldClose),
+    recentLegacyUse: Array.isArray(raw.recentLegacyUse) ? parseRecent(raw.recentLegacyUse) : null,
   };
 }
 
+function parseRecent(raw: unknown[]): RecentUse[] {
+  return raw.flatMap((entry) => {
+    if (!entry || typeof entry !== 'object') return [];
+    const r = entry as Record<string, unknown>;
+    if (typeof r.name !== 'string' || typeof r.protocol !== 'string' || typeof r.lastUsedAt !== 'number') return [];
+    return [
+      {
+        accountId: typeof r.accountId === 'string' ? r.accountId : '',
+        name: r.name,
+        protocol: r.protocol,
+        lastUsedAt: r.lastUsedAt,
+      },
+    ];
+  });
+}
+
+const PROTOCOL_LABELS: Record<string, string> = {
+  imap: 'IMAP',
+  pop3: 'POP3',
+  manageSieve: 'ManageSieve',
+  submission: 'SMTP submission',
+};
+
+/**
+ * The impact panel's lines (LP-15): one per account, naming every protocol it
+ * used and when it last used any, most recent first.
+ */
+export function impactEntries(recent: RecentUse[]): ImpactEntry[] {
+  const byAccount = new Map<string, ImpactEntry>();
+  for (const use of recent) {
+    const key = use.accountId || use.name;
+    const entry = byAccount.get(key) ?? { name: use.name, protocols: [], lastUsedAt: 0 };
+    const label = PROTOCOL_LABELS[use.protocol] ?? use.protocol;
+    if (!entry.protocols.includes(label)) entry.protocols.push(label);
+    entry.lastUsedAt = Math.max(entry.lastUsedAt, use.lastUsedAt);
+    byAccount.set(key, entry);
+  }
+  const order = Object.values(PROTOCOL_LABELS);
+  return [...byAccount.values()]
+    .map((e) => ({ ...e, protocols: e.protocols.sort((a, b) => order.indexOf(a) - order.indexOf(b)) }))
+    .sort((a, b) => b.lastUsedAt - a.lastUsedAt || a.name.localeCompare(b.name));
+}
+
+/** "2 days ago", "3 hours ago", "just now", in the reader's language. */
+export function ago(at: number, now: number, locale?: string): string {
+  const seconds = Math.round((at - now) / 1000);
+  const rtf = new Intl.RelativeTimeFormat(locale, { numeric: 'auto' });
+  const steps: [Intl.RelativeTimeFormatUnit, number][] = [
+    ['day', 86400],
+    ['hour', 3600],
+    ['minute', 60],
+  ];
+  for (const [unit, size] of steps) {
+    if (Math.abs(seconds) >= size) return rtf.format(Math.round(seconds / size), unit);
+  }
+  return rtf.format(0, 'minute');
+}
+
 /** Thrown when the server has no `inbuxa:ProtocolPolicy`, so callers can stay quiet about it. */
 export class PolicyUnavailable extends Error {}
 
 export async function fetchProtocolPolicy(signal?: AbortSignal): Promise<ProtocolPolicy> {
   const accountId = getAccountId('x:NetworkListener');
+  // No `properties`: the server answers with all of them, recentLegacyUse
+  // included where it has it.
   const responses = await jmapRequest([[`${OBJECT}/get`, { accountId, ids: null }, '0']], signal, [INBUXA_CAPABILITY]);
   const [name, result] = responses[0] ?? [];
   if (name !== `${OBJECT}/get`) {