diff --git a/packages/server/src/utils/federationPeerActivation.test.ts b/packages/server/src/utils/federationPeerActivation.test.ts
new file mode 100644
index 00000000..04b77809
--- /dev/null
+++ b/packages/server/src/utils/federationPeerActivation.test.ts
@@ -0,0 +1,95 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import Database from 'better-sqlite3';
+import { drizzle } from 'drizzle-orm/better-sqlite3';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import * as schema from '../db/schema.js';
+import { eq } from 'drizzle-orm';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let sqlite: Database.Database;
+let testDb: TestDb;
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  schema,
+}));
+
+function applyMigrations(db: Database.Database): void {
+  const migrationsDir = path.resolve(__dirname, '../../drizzle');
+  const files = fs.readdirSync(migrationsDir).filter(f => f.endsWith('.sql')).sort();
+  for (const f of files) {
+    const sql = fs.readFileSync(path.join(migrationsDir, f), 'utf8');
+    const statements = sql.split(/-->\s*statement-breakpoint/);
+    for (const stmt of statements) {
+      const clean = stmt.trim();
+      if (clean) db.exec(clean);
+    }
+  }
+}
+
+function seedPeer(id: string, status: string, lastSyncedAt = 0): void {
+  testDb.insert(schema.federationPeers).values({
+    id, origin: `https://${id}.example`, hmacSecret: 'secret',
+    status, lastSyncedAt, createdAt: Date.now(),
+  }).run();
+}
+
+function seedOutboxEntry(id: string, peerId: string, nextRetryAt: number, attempts: number): void {
+  testDb.insert(schema.federationOutbox).values({
+    id, peerId, contextId: 'ch-1', entityId: `msg-${id}`,
+    contextType: 'dm', eventType: 'create', payload: '{}',
+    encryptionVersion: 0, attempts, nextRetryAt,
+    expiresAt: Date.now() + 30 * 86_400_000,
+    createdAt: Date.now(),
+  }).run();
+}
+
+describe('resetOutboxBackoff', () => {
+  beforeEach(() => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+  });
+
+  it('resets nextRetryAt=now and attempts=0 for all peer entries — including past-due ones', async () => {
+    const { resetOutboxBackoff } = await import('./federationPeerActivation.js');
+    seedPeer('peer-a', 'active');
+    seedPeer('peer-b', 'active');
+    const now = Date.now();
+
+    // Three entries for peer-a: past-due (already eligible), near-future, far-future
+    seedOutboxEntry('entry-1', 'peer-a', now - 1000, 5);
+    seedOutboxEntry('entry-2', 'peer-a', now + 60_000, 3);
+    seedOutboxEntry('entry-3', 'peer-a', now + 86_400_000, 7);
+    // Entry for unrelated peer-b (must NOT be touched)
+    seedOutboxEntry('entry-4', 'peer-b', now + 86_400_000, 9);
+
+    resetOutboxBackoff('peer-a');
+
+    const a1 = testDb.select().from(schema.federationOutbox).where(eq(schema.federationOutbox.id, 'entry-1')).get();
+    const a2 = testDb.select().from(schema.federationOutbox).where(eq(schema.federationOutbox.id, 'entry-2')).get();
+    const a3 = testDb.select().from(schema.federationOutbox).where(eq(schema.federationOutbox.id, 'entry-3')).get();
+    const b4 = testDb.select().from(schema.federationOutbox).where(eq(schema.federationOutbox.id, 'entry-4')).get();
+
+    // All peer-a entries reset — including the past-due one (correctness: attempts=0 on those too)
+    expect(a1?.attempts).toBe(0);
+    expect(a2?.attempts).toBe(0);
+    expect(a3?.attempts).toBe(0);
+    expect(a1?.nextRetryAt).toBeGreaterThanOrEqual(now);
+    expect(a2?.nextRetryAt).toBeLessThanOrEqual(Date.now());
+    expect(a3?.nextRetryAt).toBeLessThanOrEqual(Date.now());
+    // peer-b untouched
+    expect(b4?.attempts).toBe(9);
+    expect(b4?.nextRetryAt).toBe(now + 86_400_000);
+  });
+
+  it('is a no-op when the peer has no outbox entries', async () => {
+    const { resetOutboxBackoff } = await import('./federationPeerActivation.js');
+    seedPeer('peer-empty', 'active');
+    expect(() => resetOutboxBackoff('peer-empty')).not.toThrow();
+  });
+});
diff --git a/packages/server/src/utils/federationPeerActivation.ts b/packages/server/src/utils/federationPeerActivation.ts
index d4ffef18..4c2c2c8c 100644
--- a/packages/server/src/utils/federationPeerActivation.ts
+++ b/packages/server/src/utils/federationPeerActivation.ts
@@ -52,8 +52,16 @@ export async function onPeerActivated(
  * Unconditional across all entries of the peer — see spec §Invariant 1.
  */
 export function resetOutboxBackoff(peerId: string): void {
-  // Stub — implemented in Task 2.
-  void peerId;
+  const db = getDb();
+  const now = Date.now();
+  const result = db
+    .update(schema.federationOutbox)
+    .set({ nextRetryAt: now, attempts: 0 })
+    .where(eq(schema.federationOutbox.peerId, peerId))
+    .run();
+  if (result.changes > 0) {
+    console.log(`[federation] Reset backoff on ${result.changes} outbox entries for peer ${peerId}`);
+  }
 }
 
 /**