diff --git a/src/components/settings/VacationSettings.tsx b/src/components/settings/VacationSettings.tsx
index 4af40e77..42772a56 100644
--- a/src/components/settings/VacationSettings.tsx
+++ b/src/components/settings/VacationSettings.tsx
@@ -15,6 +15,8 @@ import { useLocaleStore } from '../../stores/locale-store';
 import { useSettingsStore } from '../../stores/settings-store';
 import { isValidEmail } from '../../lib/recipients';
 import { ownDomains } from '../../lib/sieve/vacation-audience';
+import { vacationForwardAddress } from '../../lib/sieve/vacation-forward';
+import { toUnicodeEmail } from '../../lib/idn';
 import type { VacationForward } from '../../lib/sieve/types';
 import { htmlToPlainText } from '../../lib/compose-html';
 import { stripDangerousTags, escapeHtml } from '../../lib/email-html';
@@ -65,7 +67,8 @@ export function VacationSettings() {
   // Seed the editor once per load; RichTextEditor only reads initialHtml on mount.
   const [editorSeed, setEditorSeed] = useState(store.htmlBody || '');
   const [forwardEnabled, setForwardEnabled] = useState(store.forward?.enabled ?? false);
-  const [forwardTo, setForwardTo] = useState(store.forward?.to ?? '');
+  // Shown with its domain in Unicode, as it was typed.
+  const [forwardTo, setForwardTo] = useState(toUnicodeEmail(store.forward?.to ?? ''));
   const [forwardKeep, setForwardKeep] = useState(store.forward?.keepCopy ?? false);
   const [audienceOnly, setAudienceOnly] = useState<AudienceChoice>(store.audience?.only ?? 'all');
   // "Internal" are the account's own domains, as its identities give them.
@@ -92,7 +95,7 @@ export function VacationSettings() {
     setHtmlBody(store.htmlBody || '');
     setEditorSeed(store.htmlBody || '');
     setForwardEnabled(store.forward?.enabled ?? false);
-    setForwardTo(store.forward?.to ?? '');
+    setForwardTo(toUnicodeEmail(store.forward?.to ?? ''));
     setForwardKeep(store.forward?.keepCopy ?? false);
     setAudienceOnly(store.audience?.only ?? 'all');
   }, [
@@ -115,7 +118,8 @@ export function VacationSettings() {
 
   // Forwarding as a save stores it: an address that is off is kept for next
   // time if it is usable, and dropped otherwise.
-  const forwardAddress = forwardTo.trim();
+  // Checked and stored with a Unicode domain in ASCII form (webmail #1100).
+  const forwardAddress = vacationForwardAddress(forwardTo);
   const forwardInvalid = store.forwardAvailable && forwardEnabled && !isValidEmail(forwardAddress);
   const forwardSettings: VacationForwardSettings | null | undefined = !store.forwardAvailable
     ? undefined
diff --git a/src/lib/sieve/__tests__/vacation-forward-idn.test.ts b/src/lib/sieve/__tests__/vacation-forward-idn.test.ts
new file mode 100644
index 00000000..68bd407a
--- /dev/null
+++ b/src/lib/sieve/__tests__/vacation-forward-idn.test.ts
@@ -0,0 +1,29 @@
+import { describe, it, expect } from 'vitest';
+import fs from 'node:fs';
+import path from 'node:path';
+import { generateScript } from '../generator';
+import { isValidVacationForward, vacationForwardAddress } from '../vacation-forward';
+
+// A forward address with a Unicode domain went into the Sieve redirect as
+// typed; the server needs the ASCII (xn--) form (bulwarkmail/webmail#1100).
+
+describe('vacation forwarding to a Unicode domain', () => {
+  it('writes the redirect with the domain in ASCII form', () => {
+    const to = vacationForwardAddress('  Ich@bücher.de ');
+    expect(to).toBe('Ich@xn--bcher-kva.de');
+    const forward = { enabled: true, to, keepCopy: false };
+    expect(isValidVacationForward(forward)).toBe(true);
+    const script = generateScript([], undefined, { includeVacation: true, vacationForward: forward });
+    expect(script).toContain('redirect "Ich@xn--bcher-kva.de";');
+    expect(script).not.toContain('bücher');
+    // ASCII addresses stay as they are.
+    expect(vacationForwardAddress('a@example.org')).toBe('a@example.org');
+  });
+
+  // No RN renderer in the test env: check the settings page's wiring.
+  it('is what the out-of-office page checks and saves', () => {
+    const page = fs.readFileSync(path.resolve(__dirname, '../../../components/settings/VacationSettings.tsx'), 'utf8');
+    expect(page).toMatch(/const forwardAddress = vacationForwardAddress\(forwardTo\);/);
+    expect(page).toMatch(/\{ enabled: forwardEnabled, to: forwardAddress, keepCopy: forwardKeep \}/);
+  });
+});
diff --git a/src/lib/sieve/vacation-forward.ts b/src/lib/sieve/vacation-forward.ts
index 1c1d6e8c..bce8bcb6 100644
--- a/src/lib/sieve/vacation-forward.ts
+++ b/src/lib/sieve/vacation-forward.ts
@@ -1,6 +1,7 @@
 import type { VacationForward } from './types';
 import { isValidEmail } from '../recipients';
 import { isPeriodBoundary } from './period';
+import { toAsciiEmail } from '../idn';
 
 /**
  * Forwarding from the vacation card (see VacationForward). The generator
@@ -15,6 +16,15 @@ export const VACATION_FORWARD_MARKER = '# Vacation forwarding';
  */
 export const VACATION_FORWARD_MARKER_RE = /^[ \t]*# Vacation forwarding[ \t]*$/m;
 
+/**
+ * The forwarding address as typed, the way the Sieve redirect needs it: a
+ * Unicode domain in its ASCII (`xn--`) form, which the server can deliver
+ * to (webmail #1100).
+ */
+export function vacationForwardAddress(typed: string): string {
+  return toAsciiEmail(typed.trim());
+}
+
 /** Whether `value` is forwarding Bulwark can write back as it reads. */
 export function isValidVacationForward(value: unknown): value is VacationForward {
   if (!value || typeof value !== 'object') return false;