diff --git a/lib/__tests__/jmap-imip-message-id.test.ts b/lib/__tests__/jmap-imip-message-id.test.ts
new file mode 100644
index 000000000..00d41b5a6
--- /dev/null
+++ b/lib/__tests__/jmap-imip-message-id.test.ts
@@ -0,0 +1,106 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { JMAPClient } from '../jmap/client';
+import type { CalendarEvent } from '../jmap/types';
+
+// iMIP mail is created with its own Email/set; without a messageId the server
+// synthesizes one from its hostname, which on a multi-domain server names
+// another domain. It gets the same sender-domain Message-ID as other mail.
+
+type MethodCall = [string, Record<string, unknown>, string];
+
+function createClient(): JMAPClient {
+  const client = new JMAPClient('https://jmap.example.com', 'user@example.com', 'pass');
+  Object.assign(client, {
+    apiUrl: 'https://jmap.example.com/api',
+    accountId: 'account-1',
+    username: 'user@example.com',
+  });
+  return client;
+}
+
+/** Answers the requests the iMIP senders make and records each created Email. */
+function mockServer() {
+  const emails: Array<Record<string, unknown>> = [];
+  vi.spyOn(globalThis, 'fetch').mockImplementation(async (_url, init) => {
+    const { methodCalls } = JSON.parse((init as { body: string }).body) as { methodCalls: MethodCall[] };
+    const methodResponses = methodCalls.map(([name, args, callId]) => {
+      if (name === 'Mailbox/get') {
+        return [name, { list: [{ id: 'mb-drafts', name: 'Drafts', role: 'drafts' }, { id: 'mb-sent', name: 'Sent', role: 'sent' }] }, callId];
+      }
+      if (name === 'Identity/get') {
+        return [name, { list: [{ id: 'identity-1', email: 'organizer@example.org' }, { id: 'identity-2', email: 'guest@example.net' }] }, callId];
+      }
+      if (name === 'Email/set') {
+        const create = (args.create ?? {}) as Record<string, Record<string, unknown>>;
+        emails.push(...Object.values(create));
+        return [name, { created: Object.fromEntries(Object.keys(create).map((id) => [id, { id: 'email-1' }])) }, callId];
+      }
+      if (name === 'EmailSubmission/set') {
+        const created = Object.keys((args.create ?? {}) as object);
+        return [name, { created: Object.fromEntries(created.map((id) => [id, { id: 'sub-1' }])) }, callId];
+      }
+      return ['error', { type: 'unknownMethod' }, callId];
+    });
+    const payload = { methodResponses };
+    return {
+      ok: true,
+      status: 200,
+      text: () => Promise.resolve(JSON.stringify(payload)),
+      json: () => Promise.resolve(payload),
+    } as Response;
+  });
+  return emails;
+}
+
+const EVENT = {
+  id: 'event-1',
+  uid: 'uid-1@example.org',
+  title: 'Meet',
+  start: '2026-10-20T10:00:00',
+  duration: 'PT1H',
+  timeZone: 'Europe/London',
+  participants: {
+    o: { email: 'organizer@example.org', name: 'Organizer', roles: { owner: true, attendee: true } },
+    g: { email: 'guest@example.net', name: 'Guest', roles: { attendee: true } },
+  },
+} as unknown as CalendarEvent;
+
+function expectMessageIdIn(email: Record<string, unknown> | undefined, domain: string) {
+  const ids = email?.messageId as string[] | undefined;
+  expect(ids).toHaveLength(1);
+  expect(ids![0]).toMatch(new RegExp(`^[^<>@\\s]+@${domain.replace(/\./g, '\\.')}$`));
+}
+
+describe('iMIP Message-ID', () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('sets it in the organizer\'s domain on invitations', async () => {
+    const emails = mockServer();
+    await createClient().sendImipInvitation(EVENT);
+    expect(emails).toHaveLength(1);
+    expectMessageIdIn(emails[0], 'example.org');
+  });
+
+  it('sets it in the organizer\'s domain on cancellations', async () => {
+    const emails = mockServer();
+    await createClient().sendImipCancellation({ ...EVENT, status: 'cancelled' } as CalendarEvent);
+    expect(emails).toHaveLength(1);
+    expectMessageIdIn(emails[0], 'example.org');
+  });
+
+  it('sets it in the attendee\'s domain on replies', async () => {
+    const emails = mockServer();
+    await createClient().sendImipReply({
+      organizerEmail: 'organizer@example.org',
+      attendeeEmail: 'guest@example.net',
+      uid: 'uid-1@example.org',
+      summary: 'Meet',
+      dtStart: '2026-10-20T10:00:00',
+      status: 'ACCEPTED',
+    });
+    expect(emails).toHaveLength(1);
+    expectMessageIdIn(emails[0], 'example.net');
+  });
+});
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index 5feca38d8..da90a1cf7 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -4457,6 +4457,7 @@ export class JMAPClient implements IJMAPClient {
       from: [{ name: opts.attendeeName || undefined, email: opts.attendeeEmail }],
       to: [{ name: opts.organizerName || undefined, email: opts.organizerEmail }],
       subject,
+      messageId: [generateMessageId(opts.attendeeEmail)],
       keywords: { "$seen": true, "$draft": true },
       mailboxIds: { [draftsMailbox.id]: true },
       bodyStructure: {
@@ -4649,6 +4650,7 @@ export class JMAPClient implements IJMAPClient {
       from: [{ name: organizerName || undefined, email: organizerEmail }],
       to: toAddresses,
       subject,
+      messageId: [generateMessageId(organizerEmail)],
       keywords: { "$seen": true, "$draft": true },
       mailboxIds: { [draftsMailbox.id]: true },
       bodyStructure: {
@@ -4805,6 +4807,7 @@ export class JMAPClient implements IJMAPClient {
       from: [{ name: organizerName || undefined, email: organizerEmail }],
       to: toAddresses,
       subject,
+      messageId: [generateMessageId(organizerEmail)],
       keywords: { "$seen": true, "$draft": true },
       mailboxIds: { [draftsMailbox.id]: true },
       bodyStructure: {