diff --git a/lib/__tests__/demo-client-threading.test.ts b/lib/__tests__/demo-client-threading.test.ts
new file mode 100644
index 000000000..a30ed4489
--- /dev/null
+++ b/lib/__tests__/demo-client-threading.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, it } from 'vitest';
+import { DemoJMAPClient } from '@/lib/demo/demo-client';
+import { computeReplyThreadingHeaders } from '@/lib/email-threading';
+
+// A server threads a reply by In-Reply-To and References. The demo gave every
+// sent message a thread of its own, so a (quick) reply said "sent" but never
+// showed up in the conversation it answered.
+describe('DemoJMAPClient reply threading', () => {
+  async function inboxMessage(client: DemoJMAPClient) {
+    const inbox = (await client.getMailboxes()).find((m) => m.role === 'inbox')!;
+    const { emails } = await client.getEmails(inbox.id);
+    return emails.find((e) => e.messageId)!;
+  }
+
+  it('puts a sent reply into the thread of the message it answers', async () => {
+    const client = new DemoJMAPClient();
+    const original = await inboxMessage(client);
+    const threading = computeReplyThreadingHeaders(original)!;
+
+    const { emailId } = await client.sendEmail(
+      [original.from![0].email], `Re: ${original.subject}`, 'Thanks!',
+      undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined,
+      threading.inReplyTo, threading.references,
+    );
+
+    const sent = await client.getEmail(emailId!);
+    expect(sent?.threadId).toBe(original.threadId);
+    expect((await client.getThreadEmails(original.threadId)).map((e) => e.id)).toContain(emailId);
+  });
+
+  it('keeps a saved reply draft in the thread too', async () => {
+    const client = new DemoJMAPClient();
+    const original = await inboxMessage(client);
+    const threading = computeReplyThreadingHeaders(original)!;
+
+    const draftId = await client.createDraft(
+      [original.from![0].email], `Re: ${original.subject}`, 'Draft',
+      undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined,
+      threading.inReplyTo, threading.references,
+    );
+
+    const draft = await client.getEmail(draftId);
+    expect(draft?.threadId).toBe(original.threadId);
+    expect(draft?.inReplyTo).toEqual(threading.inReplyTo);
+  });
+
+  it('starts a new thread for a message that answers nothing', async () => {
+    const client = new DemoJMAPClient();
+    const original = await inboxMessage(client);
+
+    const { emailId } = await client.sendEmail(['a@example.com'], original.subject ?? 'Hello', 'New message');
+
+    const sent = await client.getEmail(emailId!);
+    expect(sent?.threadId).toBeTruthy();
+    expect(sent?.threadId).not.toBe(original.threadId);
+  });
+});
diff --git a/lib/demo/demo-client.ts b/lib/demo/demo-client.ts
index 2d098ed69..9d3139013 100644
--- a/lib/demo/demo-client.ts
+++ b/lib/demo/demo-client.ts
@@ -13,6 +13,7 @@ import { withBasePath } from '@/lib/browser-navigation';
 import { toInertBlob } from '@/lib/file-preview';
 import type { StorageQuota } from "@/lib/quota";
 import { buildDemoEml, demoEmlBlobId, demoEmlEmailId } from './demo-eml';
+import { stripMessageIdBrackets } from '@/lib/email-threading';
 
 // Fixture blobs with real bytes behind them, served from public/demo/ (see
 // fixtures/assets.ts for the list and the licences). Any other blob id falls
@@ -791,13 +792,15 @@ export class DemoJMAPClient implements IJMAPClient {
     attachments?: Array<{ blobId: string; name: string; type: string; size: number; disposition?: 'attachment' | 'inline'; cid?: string }>,
     _fromName?: string,
     htmlBody?: string,
+    inReplyTo?: string[],
+    references?: string[],
   ): Promise<string> {
     const draftsMb = this.data.mailboxes.find(m => m.role === 'drafts');
     const id = draftId || generateDemoId('email');
     const existing = draftId ? this.data.emails.findIndex(e => e.id === draftId) : -1;
 
     const email: Email = {
-      id, threadId: generateDemoId('thread'),
+      id, threadId: this.threadFor(inReplyTo, references),
       mailboxIds: { [draftsMb?.id || this.data.mailboxes[0].id]: true },
       keywords: { $seen: true, $draft: true },
       size: body.length,
@@ -819,6 +822,8 @@ export class DemoJMAPClient implements IJMAPClient {
         : { '1': { value: body } },
       attachments: attachments?.map(a => ({ ...a, partId: generateDemoId('part') })),
       messageId: `<${id}@demo.example.com>`,
+      inReplyTo: inReplyTo?.length ? inReplyTo : undefined,
+      references: references?.length ? references : undefined,
     };
 
     if (existing >= 0) {
@@ -853,7 +858,7 @@ export class DemoJMAPClient implements IJMAPClient {
     }
     const sentMb = this.data.mailboxes.find(m => m.role === 'sent');
     const email: Email = {
-      id: generateDemoId('email'), threadId: generateDemoId('thread'),
+      id: generateDemoId('email'), threadId: this.threadFor(inReplyTo, references),
       mailboxIds: { [delayedUntil ? (this.data.mailboxes.find(m => m.role === 'drafts')?.id || this.data.mailboxes[0].id) : (sentMb?.id || this.data.mailboxes[0].id)]: true },
       keywords: delayedUntil ? { $seen: true, $draft: true } : { $seen: true },
       size: body.length + (htmlBody?.length || 0),
@@ -893,6 +898,19 @@ export class DemoJMAPClient implements IJMAPClient {
       : { scheduled: false, emailId: email.id };
   }
 
+  /**
+   * The thread a new message joins. A server threads a reply by In-Reply-To
+   * and References, so the reply shows up in the conversation it answers;
+   * without a known parent the message starts a thread of its own.
+   */
+  private threadFor(inReplyTo?: string[], references?: string[]): string {
+    const wanted = new Set([...(inReplyTo ?? []), ...(references ?? [])].map(stripMessageIdBrackets));
+    const parent = wanted.size > 0
+      ? this.data.emails.find(e => e.messageId && wanted.has(stripMessageIdBrackets(e.messageId)))
+      : undefined;
+    return parent?.threadId ?? generateDemoId('thread');
+  }
+
   /** The From address of new mail: the chosen identity, else the account's own address. */
   private fromAddress(identityId?: string, fromEmail?: string, fromName?: string): EmailAddress {
     const identity = identityId ? this.data.identities.find(i => i.id === identityId) : undefined;