From dcbfc907fae782eb5f1026263bbd0c5b90cd7f24 Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Sat, 31 Oct 2026 04:05:57 +0100
Subject: [PATCH] feat: warn about the server's forward limit and an auto-reply
 too long for it (bulwarkmail/webmail#1155, bulwarkmail/webmail#1208)

---
 src/api/sieve-rules.ts                        |  10 ++
 src/components/filters/FilterRuleModal.tsx    |  43 ++++-
 src/components/filters/QuickRuleSheet.tsx     |  27 ++-
 src/components/settings/VacationSettings.tsx  |  40 ++++-
 src/lib/__tests__/vacation-limits.test.ts     |  28 +++
 .../__tests__/forward-limit-editor.test.ts    |  62 +++++++
 .../__tests__/forward-limit-vacation.test.ts  |  59 +++++++
 src/lib/sieve/__tests__/forward-limit.test.ts | 162 ++++++++++++++++++
 src/lib/sieve/forward-limit.ts                | 136 +++++++++++++++
 src/lib/vacation-limits.ts                    |  29 ++++
 src/stores/__tests__/filter-store.test.ts     |  11 ++
 src/stores/__tests__/vacation-store.test.ts   |   2 +-
 src/stores/filter-store.ts                    |   8 +
 src/stores/vacation-store.ts                  |   4 +
 14 files changed, 614 insertions(+), 7 deletions(-)
 create mode 100644 src/lib/__tests__/vacation-limits.test.ts
 create mode 100644 src/lib/sieve/__tests__/forward-limit-editor.test.ts
 create mode 100644 src/lib/sieve/__tests__/forward-limit-vacation.test.ts
 create mode 100644 src/lib/sieve/__tests__/forward-limit.test.ts
 create mode 100644 src/lib/sieve/forward-limit.ts
 create mode 100644 src/lib/vacation-limits.ts

diff --git a/src/api/sieve-rules.ts b/src/api/sieve-rules.ts
index 672faf77..03ad09df 100644
--- a/src/api/sieve-rules.ts
+++ b/src/api/sieve-rules.ts
@@ -33,6 +33,16 @@ export class FiltersLoginChangedError extends Error {
   }
 }
 
+/** The account's filters as the server has them now (its active script, parsed). */
+export async function readAccountFilters(accountId: string = getSieveAccountId()): Promise<ParseResult> {
+  const all = await getSieveScripts(accountId);
+  const scripts = all.filter((s) => s.name !== VACATION_SCRIPT_NAME);
+  const script = scripts.find((s) => s.isActive) || scripts[0] || null;
+  return script
+    ? parseScript(await getSieveScriptContent(script.blobId, accountId))
+    : { rules: [], isOpaque: false, externalRequires: [] };
+}
+
 /**
  * Read the account's script from the server, let `modify` change its rules,
  * and write the result back as the active script, keeping the vacation
diff --git a/src/components/filters/FilterRuleModal.tsx b/src/components/filters/FilterRuleModal.tsx
index 503cf800..7968b597 100644
--- a/src/components/filters/FilterRuleModal.tsx
+++ b/src/components/filters/FilterRuleModal.tsx
@@ -42,6 +42,8 @@ import type {
   FilterActionType,
 } from '../../lib/sieve/types';
 import { SafeAreaModal } from '../SafeAreaModal';
+import { useFilterStore } from '../../stores/filter-store';
+import { forwardsOfEditedRule, ruleForwardLimit } from '../../lib/sieve/forward-limit';
 import { MODAL_ORIENTATIONS } from '../../lib/modal-orientations';
 
 const ALL_FIELDS: FilterConditionField[] = ['from', 'to', 'cc', 'subject', 'header', 'size', 'body', 'attachment', 'all'];
@@ -72,12 +74,19 @@ interface FilterRuleModalProps {
   mailboxes: Mailbox[];
   /** The server can confine a rule to a period (Sieve "date" and "relational"). */
   periodsSupported?: boolean;
+  /**
+   * The server's cap on forwards per message, and the forwards a message can
+   * have collected when it reaches this rule / still collect below it (see
+   * lib/sieve/forward-limit.ts). Left out: read from the filter list the
+   * Filters settings show.
+   */
+  forwardContext?: { maxRedirects: number | null | undefined; before: number; after: number };
   onSave: (rule: FilterRule) => void;
   onClose: () => void;
 }
 
 export function FilterRuleModal({
-  visible, rule: editedRule, initialRule, mailboxes, periodsSupported = false, onSave, onClose,
+  visible, rule: editedRule, initialRule, mailboxes, periodsSupported = false, forwardContext, onSave, onClose,
 }: FilterRuleModalProps) {
   const c = useColors();
   const styles = useMemo(() => makeStyles(c), [c]);
@@ -120,6 +129,25 @@ export function FilterRuleModal({
 
   const mailboxTargets = useMemo(() => buildMailboxTargets(mailboxes), [mailboxes]);
 
+  // A message can collect only so many forwards on the server (#1155, #1208).
+  const listedRules = useFilterStore((s) => s.rules);
+  const listedForward = useFilterStore((s) => s.vacationForward);
+  const listedMaxRedirects = useFilterStore((s) => s.sieveCapabilities?.maxNumberRedirects);
+  const listedAround = useMemo(
+    () => forwardsOfEditedRule(listedRules, listedForward, editedRule?.id),
+    [listedRules, listedForward, editedRule?.id],
+  );
+  const forwardLimit = ruleForwardLimit(
+    { actions, stopProcessing },
+    forwardContext ? forwardContext.maxRedirects : listedMaxRedirects,
+    forwardContext ?? listedAround,
+  );
+  const ruleForwardCount = actions.filter((a) => a.type === 'forward').length;
+  const actionOptionsFor = (action: FilterAction) =>
+    action.type === 'forward' || forwardLimit.canAddForward
+      ? actionTypeOptions
+      : actionTypeOptions.filter((o) => o.value !== 'forward');
+
   const fieldOptions = useMemo(
     () => ALL_FIELDS.map((f) => ({ value: f, label: t(`settings.filters.condition_fields.${f}`, f) })),
     [t],
@@ -407,7 +435,7 @@ export function FilterRuleModal({
                       <Select
                         value={action.type}
                         onChange={(v) => updateAction(index, { type: v as FilterActionType })}
-                        options={actionTypeOptions}
+                        options={actionOptionsFor(action)}
                         style={{ flex: 1 }}
                       />
                       <Pressable
@@ -489,6 +517,16 @@ export function FilterRuleModal({
                   </View>
                 ))}
               </View>
+              {forwardLimit.unavailable && ruleForwardCount > 0 && (
+                <Text style={styles.forwardWarning}>
+                  {t('settings.filters.forward_unavailable', 'This server does not allow forwarding. Forward actions in filter rules have no effect.')}
+                </Text>
+              )}
+              {forwardLimit.overLimit && (
+                <Text style={styles.forwardWarning}>
+                  {t('settings.filters.forward_limit', 'Forward limit per message on this server: {count}. Extra forwards are skipped.', { count: forwardLimit.limit ?? 0 })}
+                </Text>
+              )}
               <Pressable
                 onPress={() => setActions((prev) => [...prev, makeEmptyAction()])}
                 style={styles.addRow}
@@ -645,6 +683,7 @@ function makeStyles(c: ThemePalette) {
     },
     optionLabel: { ...typography.body, color: c.text, flex: 1 },
     hint: { ...typography.caption, color: c.mutedForeground },
+    forwardWarning: { ...typography.caption, color: c.warning, marginTop: spacing.sm },
 
     footer: {
       flexDirection: 'row',
diff --git a/src/components/filters/QuickRuleSheet.tsx b/src/components/filters/QuickRuleSheet.tsx
index 316d96b4..94c1067a 100644
--- a/src/components/filters/QuickRuleSheet.tsx
+++ b/src/components/filters/QuickRuleSheet.tsx
@@ -5,7 +5,8 @@ import { MoveSheet } from '../MoveSheet';
 import { FilterRuleModal } from './FilterRuleModal';
 import type { Email, Mailbox } from '../../api/types';
 import { getSieveAccountId, getSieveCapabilities } from '../../api/sieve';
-import { FiltersLoginChangedError, OpaqueFiltersError, updateAccountFilters } from '../../api/sieve-rules';
+import { FiltersLoginChangedError, OpaqueFiltersError, readAccountFilters, updateAccountFilters } from '../../api/sieve-rules';
+import { forwardsAround, inRunOrder } from '../../lib/sieve/forward-limit';
 import { jmapClient } from '../../api/jmap-client';
 import { servedLoginOf } from '../../lib/draft-login';
 import { supportsPeriods } from '../../lib/sieve/period';
@@ -87,6 +88,26 @@ export function QuickRuleSheet({ visible, email, mailboxes, onClose }: QuickRule
   const junk = React.useMemo(() => findJunkMailbox(mailboxes), [mailboxes]);
   const periodsSupported = supportsPeriods(getSieveCapabilities()?.sieveExtensions);
 
+  // Where the new rule runs among the account's rules, for the server's
+  // redirect limit: at the top, behind the out of office forwarding
+  // (webmail quick-rule-host.tsx; lib/sieve/forward-limit.ts).
+  const editorOpen = visible && step.kind === 'editor';
+  const [forwards, setForwards] = React.useState({ before: 0, after: 0 });
+  React.useEffect(() => {
+    setForwards({ before: 0, after: 0 });
+    if (!editorOpen) return;
+    let current = true;
+    const accountId = openedIn.current.accountId ?? undefined;
+    readAccountFilters(accountId)
+      .then((parsed) => {
+        if (!current || parsed.isOpaque) return;
+        const runOrder = inRunOrder(parsed.rules, parsed.vacationForward);
+        setForwards(forwardsAround(runOrder, runOrder.length - parsed.rules.length, false));
+      })
+      .catch(() => undefined);
+    return () => { current = false; };
+  }, [editorOpen]);
+
   const [sender] = subject.senders;
   const senderLabel = subject.senders.length === 1
     ? (sender.name || sender.email)
@@ -248,6 +269,10 @@ export function QuickRuleSheet({ visible, email, mailboxes, onClose }: QuickRule
         initialRule={step.kind === 'editor' ? step.rule : undefined}
         mailboxes={mailboxes}
         periodsSupported={periodsSupported}
+        forwardContext={{
+          maxRedirects: getSieveCapabilities(openedIn.current.accountId ?? undefined)?.maxNumberRedirects,
+          ...forwards,
+        }}
         onSave={(rule) => void saveEditorRule(rule)}
         onClose={finish}
       />
diff --git a/src/components/settings/VacationSettings.tsx b/src/components/settings/VacationSettings.tsx
index 87686feb..4af40e77 100644
--- a/src/components/settings/VacationSettings.tsx
+++ b/src/components/settings/VacationSettings.tsx
@@ -26,6 +26,9 @@ import {
   parseLocalInput,
   utcIsoToLocalInput,
 } from '../../lib/vacation-dates';
+import { STALWART_VACATION_LIMITS, vacationOversize } from '../../lib/vacation-limits';
+import { jmapClient } from '../../api/jmap-client';
+import { getSieveCapabilities } from '../../api/sieve';
 
 // Ported from the webmail's components/settings/vacation-settings.tsx. Dates
 // are typed as local wall-clock time and sent as RFC 8621 UTCDate strings;
@@ -122,6 +125,11 @@ export function VacationSettings() {
   // Only what was changed here is sent: the rest stays as the server has it,
   // which another device may have changed since this one loaded.
   const forwardChanged = forwardSettings !== undefined && !sameForward(forwardSettings, store.forward);
+  // Kept here, the message goes on through the filter rules, whose forwards
+  // share the server's limit with this one (#1208).
+  const forwardLimit = getSieveCapabilities(managedAccountId ?? undefined)?.maxNumberRedirects;
+  const forwardOverLimit = store.forwardAvailable && forwardEnabled && forwardKeep
+    && typeof forwardLimit === 'number' && forwardLimit > 0 && 1 + store.otherForwards > forwardLimit;
 
   const fromParsed = parseLocalInput(fromDate);
   const toParsed = parseLocalInput(toDate);
@@ -137,6 +145,22 @@ export function VacationSettings() {
     return fromParsed < todayStart;
   }, [fromParsed]);
 
+  // What a save sends, measured against the server's limits as you type.
+  const sizeLimits = jmapClient.hasAccountCapability('urn:stalwart:jmap', managedAccountId ?? undefined)
+    ? STALWART_VACATION_LIMITS
+    : null;
+  const oversize = useMemo(() => {
+    const sanitizedHtml = hasHtmlContent ? stripDangerousTags(htmlBody) : null;
+    const textBody = body.trim() || !sanitizedHtml ? body : htmlToPlainText(sanitizedHtml);
+    return vacationOversize({ subject: subject.trim(), textBody, htmlBody: sanitizedHtml }, sizeLimits);
+  }, [hasHtmlContent, htmlBody, body, subject, sizeLimits]);
+  const subjectTooLong = oversize.subject && sizeLimits
+    ? t('settings.vacation.warnings.subject_too_long', 'The subject is too long for the server (at most {max} bytes).', { max: sizeLimits.subject })
+    : null;
+  const bodyTooLong = oversize.body && sizeLimits
+    ? t('settings.vacation.warnings.body_too_long', 'The message is too long for the server (at most {max} bytes, formatting included).', { max: sizeLimits.body })
+    : null;
+
   const warnings: string[] = [];
   if (endBeforeStart) warnings.push(t('settings.vacation.warnings.end_before_start', 'End date must be after start date'));
   if (startInPast) warnings.push(t('settings.vacation.warnings.start_in_past', 'Start date is in the past'));
@@ -155,7 +179,8 @@ export function VacationSettings() {
     forwardChanged ||
     audienceChanged;
 
-  const canSave = hasChanges && !endBeforeStart && !formatError && !forwardInvalid && !store.isSaving;
+  const canSave = hasChanges && !endBeforeStart && !formatError && !forwardInvalid
+    && !oversize.subject && !oversize.body && !store.isSaving;
 
   const handleSave = useCallback(async () => {
     // Read the live editor DOM rather than trusting onChange state (issue #9).
@@ -342,8 +367,9 @@ export function VacationSettings() {
             placeholder={t('settings.vacation.message.subject_placeholder', 'Out of Office')}
             placeholderTextColor={c.mutedForeground}
             accessibilityLabel={t('settings.vacation.message.subject_label', 'Subject')}
-            style={styles.subjectInput}
+            style={[styles.subjectInput, oversize.subject && styles.inputInvalid]}
           />
+          {subjectTooLong && <Text style={styles.fieldError}>{subjectTooLong}</Text>}
         </View>
         <View style={styles.bodyBlock}>
           <Text style={styles.bodyLabel}>{t('settings.vacation.message.body_label', 'Message Body')}</Text>
@@ -353,11 +379,12 @@ export function VacationSettings() {
             onChangeText={setBody}
             placeholder={t('settings.vacation.message.body_placeholder', 'Thank you for your email. I am currently out of the office and will respond when I return.')}
             placeholderTextColor={c.mutedForeground}
-            style={styles.bodyInput}
+            style={[styles.bodyInput, oversize.body && styles.inputInvalid]}
             multiline
             numberOfLines={6}
             textAlignVertical="top"
           />
+          {bodyTooLong && <Text style={styles.fieldError}>{bodyTooLong}</Text>}
         </View>
         <SettingItem
           label={t('settings.vacation.message.html_label', 'Formatted message (HTML)')}
@@ -466,6 +493,11 @@ export function VacationSettings() {
                   accessibilityLabel={t('settings.vacation.forward.keep_label', 'Keep in inbox')}
                 />
               </SettingItem>
+              {forwardOverLimit && (
+                <Text style={styles.forwardWarning}>
+                  {t('settings.filters.forward_limit', 'Forward limit per message on this server: {count}. Extra forwards are skipped.', { count: forwardLimit })}
+                </Text>
+              )}
             </>
           )}
         </SettingsSection>
@@ -570,6 +602,8 @@ function makeStyles(c: ThemePalette) {
     ...typography.body,
   },
   inputInvalid: { borderColor: c.error },
+  fieldError: { ...typography.caption, color: c.error, marginTop: spacing.xs },
+  forwardWarning: { ...typography.caption, color: c.warning, paddingHorizontal: spacing.lg, paddingBottom: spacing.md },
   subjectInput: {
     paddingHorizontal: spacing.md,
     paddingVertical: 6,
diff --git a/src/lib/__tests__/vacation-limits.test.ts b/src/lib/__tests__/vacation-limits.test.ts
new file mode 100644
index 00000000..ccaf346c
--- /dev/null
+++ b/src/lib/__tests__/vacation-limits.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, it } from 'vitest';
+import { STALWART_VACATION_LIMITS, utf8Length, vacationOversize } from '../vacation-limits';
+
+describe('vacationOversize', () => {
+  const limits = STALWART_VACATION_LIMITS;
+
+  it('flags a subject of 512 bytes and a body of 2048 bytes, not one byte less', () => {
+    expect(vacationOversize({ subject: 'a'.repeat(511), textBody: 'b'.repeat(2047), htmlBody: null }, limits))
+      .toEqual({ subject: false, body: false });
+    expect(vacationOversize({ subject: 'a'.repeat(512), textBody: 'b'.repeat(2048), htmlBody: null }, limits))
+      .toEqual({ subject: true, body: true });
+  });
+
+  it('counts bytes, not characters, and the HTML part with its formatting', () => {
+    // 256 umlauts are 512 bytes.
+    expect(vacationOversize({ subject: 'ä'.repeat(256), textBody: '', htmlBody: null }, limits).subject).toBe(true);
+    expect(vacationOversize({ subject: '', textBody: 'short', htmlBody: `<p>${'x'.repeat(2045)}</p>` }, limits).body).toBe(true);
+  });
+
+  it('says nothing for a server without known limits', () => {
+    expect(vacationOversize({ subject: 'a'.repeat(9000), textBody: '', htmlBody: null }, null))
+      .toEqual({ subject: false, body: false });
+  });
+
+  it('measures UTF-8 like the server', () => {
+    expect(utf8Length('aä€😀')).toBe(1 + 2 + 3 + 4);
+  });
+});
diff --git a/src/lib/sieve/__tests__/forward-limit-editor.test.ts b/src/lib/sieve/__tests__/forward-limit-editor.test.ts
new file mode 100644
index 00000000..91d21007
--- /dev/null
+++ b/src/lib/sieve/__tests__/forward-limit-editor.test.ts
@@ -0,0 +1,62 @@
+import { describe, expect, it } from 'vitest';
+import type { FilterAction, FilterRule, VacationForward } from '../types';
+import { forwardsOfEditedRule, ruleForwardLimit, rulesForwardLimit } from '../forward-limit';
+
+const forward: FilterAction = { type: 'forward', value: 'x@example.com' };
+const rule = (id: string, actions: FilterAction[], extra: Partial<FilterRule> = {}): FilterRule => ({
+  id,
+  name: id,
+  enabled: true,
+  matchType: 'all',
+  conditions: [{ field: 'from', comparator: 'contains', value: 'a' }],
+  actions,
+  stopProcessing: false,
+  ...extra,
+});
+const deputy: VacationForward = { enabled: true, to: 'deputy@example.com', keepCopy: true } as VacationForward;
+
+describe('ruleForwardLimit', () => {
+  it('warns when one message would collect more forwards than the server runs (Stalwart: 1)', () => {
+    const state = ruleForwardLimit({ actions: [forward], stopProcessing: false }, 1, { before: 1, after: 0 });
+    expect(state).toEqual({ limit: 1, overLimit: true, unavailable: false, canAddForward: false });
+  });
+
+  it('lets a rule that stops ignore the forwards below it', () => {
+    expect(ruleForwardLimit({ actions: [forward], stopProcessing: true }, 1, { before: 0, after: 3 }).overLimit).toBe(false);
+    expect(ruleForwardLimit({ actions: [forward], stopProcessing: false }, 1, { before: 0, after: 3 }).overLimit).toBe(true);
+  });
+
+  it('offers a forward while there is room and none on a server without forwarding', () => {
+    expect(ruleForwardLimit({ actions: [{ type: 'move', value: 'A' }], stopProcessing: false }, 1, { before: 0, after: 0 }).canAddForward).toBe(true);
+    const off = ruleForwardLimit({ actions: [forward], stopProcessing: false }, 0, { before: 0, after: 0 });
+    expect(off).toMatchObject({ limit: null, overLimit: false, unavailable: true, canAddForward: false });
+  });
+
+  it('says nothing without a cap from the server', () => {
+    expect(ruleForwardLimit({ actions: [forward, forward], stopProcessing: false }, undefined, { before: 5, after: 5 }))
+      .toEqual({ limit: null, overLimit: false, unavailable: false, canAddForward: true });
+  });
+});
+
+describe('forwardsOfEditedRule', () => {
+  const rules = [rule('a', [forward]), rule('b', [{ type: 'move', value: 'A' }]), rule('ext', [forward], { origin: 'external' })];
+
+  it('counts the forwards above and below the edited rule, the out of office forwarding first', () => {
+    expect(forwardsOfEditedRule(rules, deputy, 'b')).toEqual({ before: 2, after: 1 });
+    expect(forwardsOfEditedRule(rules, null, 'a')).toEqual({ before: 0, after: 1 });
+  });
+
+  it('puts a new rule below Bulwark\'s own rules, above the external ones', () => {
+    expect(forwardsOfEditedRule(rules, null, undefined)).toEqual({ before: 1, after: 1 });
+  });
+});
+
+describe('rulesForwardLimit', () => {
+  it('flags rules whose forwards add up past the cap, and forwards a server ignores', () => {
+    const rules = [rule('a', [forward]), rule('b', [forward])];
+    expect(rulesForwardLimit(rules, null, 1)).toEqual({ tooMany: true, ignored: false });
+    expect(rulesForwardLimit(rules, null, 2)).toEqual({ tooMany: false, ignored: false });
+    expect(rulesForwardLimit([rules[0]], deputy, 1)).toEqual({ tooMany: true, ignored: false });
+    expect(rulesForwardLimit(rules, null, 0)).toEqual({ tooMany: false, ignored: true });
+  });
+});
diff --git a/src/lib/sieve/__tests__/forward-limit-vacation.test.ts b/src/lib/sieve/__tests__/forward-limit-vacation.test.ts
new file mode 100644
index 00000000..9e7b78b1
--- /dev/null
+++ b/src/lib/sieve/__tests__/forward-limit-vacation.test.ts
@@ -0,0 +1,59 @@
+import { describe, expect, it } from 'vitest';
+import type { FilterAction, FilterRule, VacationForward } from '../types';
+import { generateScript } from '../generator';
+import { VACATION_FORWARD_MARKER } from '../vacation-forward';
+import { forwardsAround, inRunOrder, ruleForwards, ruleStops, worstCaseForwards } from '../forward-limit';
+
+const forward: FilterAction = { type: 'forward', value: 'office@example.com' };
+const rule = (actions: FilterAction[], stopProcessing = false): FilterRule => ({
+  id: `r-${Math.random()}`,
+  name: 'R',
+  enabled: true,
+  matchType: 'all',
+  conditions: [{ field: 'from', comparator: 'contains', value: 'a' }],
+  actions,
+  stopProcessing,
+});
+const vacationForward = (keepCopy: boolean, enabled = true): VacationForward => ({
+  enabled,
+  to: 'deputy@example.com',
+  keepCopy,
+});
+
+describe('the out of office forwarding in the run order', () => {
+  const rules = [rule([forward])];
+
+  it('goes ahead of the rules, and stops unless it keeps a copy', () => {
+    expect(inRunOrder(rules, vacationForward(true))).toEqual([
+      { enabled: true, actions: [{ type: 'forward', value: 'deputy@example.com' }], stopProcessing: false },
+      ...rules,
+    ]);
+    expect(inRunOrder(rules, vacationForward(false))[0].stopProcessing).toBe(true);
+  });
+
+  it('is left out while it is off', () => {
+    expect(inRunOrder(rules, vacationForward(true, false))).toBe(rules);
+    expect(inRunOrder(rules, null)).toBe(rules);
+    expect(inRunOrder(rules, undefined)).toBe(rules);
+  });
+
+  it('agrees with the script: one redirect ahead of every rule, and a stop unless a copy is kept', () => {
+    for (const keepCopy of [true, false]) {
+      const script = generateScript(rules, undefined, { vacationForward: vacationForward(keepCopy) });
+      expect(script.indexOf(VACATION_FORWARD_MARKER)).toBeGreaterThan(-1);
+      expect(script.indexOf(VACATION_FORWARD_MARKER)).toBeLessThan(script.indexOf('# Rule:'));
+      const lines = script.split(VACATION_FORWARD_MARKER)[1].split('# Rule:')[0].split('\n').map((line) => line.trim());
+      const [ahead] = inRunOrder([], vacationForward(keepCopy));
+      expect(lines.filter((line) => line.startsWith('redirect ')).length, `keepCopy ${keepCopy}`).toBe(ruleForwards(ahead));
+      expect(lines.includes('stop;'), `keepCopy ${keepCopy}`).toBe(ruleStops(ahead));
+    }
+  });
+
+  it('adds its forward to every rule only when it keeps a copy', () => {
+    // A new rule goes behind Bulwark's rules, here behind the one rule.
+    expect(forwardsAround(inRunOrder(rules, vacationForward(true)), 2, false)).toEqual({ before: 2, after: 0 });
+    expect(forwardsAround(inRunOrder(rules, vacationForward(false)), 2, false)).toEqual({ before: 1, after: 0 });
+    expect(worstCaseForwards(inRunOrder(rules, vacationForward(true)))).toBe(2);
+    expect(worstCaseForwards(inRunOrder(rules, vacationForward(false)))).toBe(1);
+  });
+});
diff --git a/src/lib/sieve/__tests__/forward-limit.test.ts b/src/lib/sieve/__tests__/forward-limit.test.ts
new file mode 100644
index 00000000..bb896ec7
--- /dev/null
+++ b/src/lib/sieve/__tests__/forward-limit.test.ts
@@ -0,0 +1,162 @@
+import { describe, expect, it } from 'vitest';
+import type { FilterAction, FilterRule } from '../types';
+import { generateScript } from '../generator';
+import { forwardsAround, ruleForwards, ruleStops, worstCaseForwards } from '../forward-limit';
+
+const forward: FilterAction = { type: 'forward', value: 'x@example.com' };
+const rule = (actions: FilterAction[], stopProcessing = false, enabled = true): FilterRule => ({
+  id: `r-${Math.random()}`,
+  name: 'R',
+  enabled,
+  matchType: 'all',
+  conditions: [{ field: 'from', comparator: 'contains', value: 'a' }],
+  actions,
+  stopProcessing,
+});
+
+// "Forward to a colleague, delete silently, stop processing", and a rule for
+// every other message that forwards too.
+const vendor = rule([forward, { type: 'discard' }], true);
+const everyoneElse = rule([forward]);
+
+describe('the forwards of one rule', () => {
+  it('counts those ahead of a Stop action', () => {
+    expect(ruleForwards(rule([forward, forward]))).toBe(2);
+    expect(ruleForwards(rule([forward, { type: 'stop' }, forward]))).toBe(1);
+    expect(ruleForwards(rule([{ type: 'mark_read' }]))).toBe(0);
+  });
+
+  it('knows a rule stops by its checkbox or a Stop action', () => {
+    expect(ruleStops(rule([forward], true))).toBe(true);
+    expect(ruleStops(rule([forward, { type: 'stop' }]))).toBe(true);
+    // Delete silently and Reject alone do not end the script.
+    expect(ruleStops(rule([forward, { type: 'discard' }]))).toBe(false);
+    expect(ruleStops(rule([{ type: 'reject', value: 'no' }]))).toBe(false);
+  });
+});
+
+describe('the most forwards one message can collect', () => {
+  it('stays at one when a rule that forwards stops ahead of another that forwards', () => {
+    expect(worstCaseForwards([vendor, everyoneElse])).toBe(1);
+  });
+
+  it('is two once the rule for every message comes first', () => {
+    // A message for the colleague meets both: the second forward would be skipped.
+    expect(worstCaseForwards([everyoneElse, vendor])).toBe(2);
+  });
+
+  it('adds up the rules that let a message go on', () => {
+    expect(worstCaseForwards([everyoneElse, rule([forward]), rule([forward])])).toBe(3);
+    expect(worstCaseForwards([rule([forward], true), rule([forward, forward], true)])).toBe(2);
+  });
+
+  it('leaves out rules that are off', () => {
+    expect(worstCaseForwards([rule([forward], false, false), vendor])).toBe(1);
+  });
+
+  it('is none without forwards', () => {
+    expect(worstCaseForwards([])).toBe(0);
+    expect(worstCaseForwards([rule([{ type: 'mark_read' }], true)])).toBe(0);
+  });
+});
+
+describe('the forwards around one rule', () => {
+  it('takes nothing from a rule above that stops, for a new rule below it', () => {
+    expect(forwardsAround([vendor], 1, false)).toEqual({ before: 0, after: 0 });
+  });
+
+  it('takes the forwards of a rule above that lets messages go on', () => {
+    const going = rule([forward, { type: 'discard' }]);
+    expect(forwardsAround([going], 1, false)).toEqual({ before: 1, after: 0 });
+  });
+
+  it('takes the worst case below an edited rule, without the rule itself', () => {
+    expect(forwardsAround([vendor, everyoneElse], 0, true)).toEqual({ before: 0, after: 1 });
+    expect(forwardsAround([everyoneElse, vendor], 1, true)).toEqual({ before: 1, after: 0 });
+  });
+
+  it('puts a new rule at the index, keeping the rule there below it', () => {
+    expect(forwardsAround([everyoneElse, vendor], 1, false)).toEqual({ before: 1, after: 1 });
+  });
+});
+
+describe('the count and the script', () => {
+  it('agree on the forwards a rule runs and whether it stops', () => {
+    const variants: FilterAction[][] = [
+      [forward],
+      [forward, { type: 'discard' }],
+      [forward, { type: 'reject', value: 'no' }],
+      [forward, { type: 'move', value: 'Archive' }],
+      [{ type: 'mark_read' }, forward, forward],
+      [forward, { type: 'stop' }, forward],
+      [{ type: 'stop' }, forward],
+    ];
+    for (const actions of variants) {
+      for (const stopProcessing of [false, true]) {
+        const r = rule(actions, stopProcessing);
+        const lines = generateScript([r]).split('# Rule:')[1].split('\n').map((line) => line.trim());
+        const stop = lines.indexOf('stop;');
+        const runs = stop === -1 ? lines : lines.slice(0, stop);
+        const label = `${actions.map((a) => a.type).join(', ')}${stopProcessing ? ', stop processing' : ''}`;
+        expect(runs.filter((line) => line.startsWith('redirect ')).length, label).toBe(ruleForwards(r));
+        expect(stop !== -1, label).toBe(ruleStops(r));
+      }
+    }
+  });
+});
+
+describe('against every set of rules a message can match', () => {
+  // A message that matches the rules in `matched`: the forwards it collects,
+  // and the rules that run for it.
+  function walk(rules: FilterRule[], matched: Set<number>): { forwards: number; ran: Set<number> } {
+    let forwards = 0;
+    const ran = new Set<number>();
+    for (let i = 0; i < rules.length; i++) {
+      if (!rules[i].enabled || !matched.has(i)) continue;
+      ran.add(i);
+      forwards += ruleForwards(rules[i]);
+      if (ruleStops(rules[i])) break;
+    }
+    return { forwards, ran };
+  }
+  // The most forwards one message can collect; with `runs`, one the rule
+  // there runs for.
+  function mostCollected(rules: FilterRule[], runs?: number): number {
+    let most = 0;
+    for (let mask = 0; mask < 1 << rules.length; mask++) {
+      const matched = new Set(rules.map((_, i) => i).filter((i) => mask & (1 << i)));
+      const { forwards, ran } = walk(rules, matched);
+      if (runs === undefined || ran.has(runs)) most = Math.max(most, forwards);
+    }
+    return most;
+  }
+
+  // Fixed seed: the same rule lists on every run.
+  let seed = 7;
+  const random = () => (seed = (seed * 16807) % 2147483647) / 2147483647;
+  const pool: FilterAction[][] = [
+    [], [forward], [forward, forward], [forward, { type: 'discard' }],
+    [forward, { type: 'stop' }], [{ type: 'stop' }, forward], [{ type: 'mark_read' }],
+  ];
+  const lists = Array.from({ length: 300 }, () =>
+    Array.from({ length: Math.floor(random() * 7) }, () =>
+      rule(pool[Math.floor(random() * pool.length)], random() < 0.4, random() < 0.8)));
+
+  it('finds the most forwards one message can collect', () => {
+    for (const rules of lists) expect(worstCaseForwards(rules)).toBe(mostCollected(rules));
+  });
+
+  it('finds the most a message the rule runs for can collect, for rules in place and new ones', () => {
+    for (const rules of lists) {
+      for (let i = 0; i < rules.length; i++) {
+        const r = rules[i];
+        if (!r.enabled) continue;
+        const { before, after } = forwardsAround(rules, i, true);
+        expect(before + ruleForwards(r) + (ruleStops(r) ? 0 : after)).toBe(mostCollected(rules, i));
+        // The same rule about to be put there, rather than there already.
+        const others = rules.filter((_, j) => j !== i);
+        expect(forwardsAround(others, i, false)).toEqual({ before, after });
+      }
+    }
+  });
+});
diff --git a/src/lib/sieve/forward-limit.ts b/src/lib/sieve/forward-limit.ts
new file mode 100644
index 00000000..fa16ab42
--- /dev/null
+++ b/src/lib/sieve/forward-limit.ts
@@ -0,0 +1,136 @@
+import type { FilterRule, VacationForward } from './types';
+
+// Ported from the webmail's lib/filters/forward-limit.ts (#1155, #1208).
+
+/*
+ * A server lets one message trigger only so many redirects
+ * (maxNumberRedirects, 1 for user scripts on Stalwart by default) and skips
+ * the ones over that. The rules run in order, and a rule that stops ("Stop
+ * processing", or the Stop action) ends the script for the messages it
+ * matches: the forwards of the rules below it never add to its own. What
+ * counts is the most forwards a single message can collect, not how many
+ * forwards there are.
+ */
+
+type RuleForwards = Pick<FilterRule, 'enabled' | 'actions' | 'stopProcessing'>;
+
+/** The forwards of a rule that run: those ahead of a Stop action. */
+export function ruleForwards(rule: Pick<FilterRule, 'actions'>): number {
+  let forwards = 0;
+  for (const action of rule.actions) {
+    if (action.type === 'stop') break;
+    if (action.type === 'forward') forwards++;
+  }
+  return forwards;
+}
+
+/** The rule ends the script for the messages it matches. */
+export function ruleStops(rule: Pick<FilterRule, 'actions' | 'stopProcessing'>): boolean {
+  return rule.stopProcessing || rule.actions.some((action) => action.type === 'stop');
+}
+
+/** The most forwards one message can collect going through `rules`, in their order. */
+export function worstCaseForwards(rules: RuleForwards[]): number {
+  // From the rules so far that let the message go on.
+  let open = 0;
+  let worst = 0;
+  for (const rule of rules) {
+    if (!rule.enabled) continue;
+    const forwards = ruleForwards(rule);
+    if (ruleStops(rule)) worst = Math.max(worst, open + forwards);
+    else open += forwards;
+  }
+  return Math.max(worst, open);
+}
+
+/**
+ * For the rule at `index` of `rules` (in the order they run): the forwards a
+ * message can have collected when it reaches the rule, and the most it can
+ * still collect below it when the rule lets it go on. `replaces`: the rule
+ * is at `index` already, rather than about to be put there.
+ */
+export function forwardsAround(
+  rules: RuleForwards[],
+  index: number,
+  replaces: boolean,
+): { before: number; after: number } {
+  // A rule above that stops lets only the messages it does not match go on,
+  // so its forwards never meet this rule's.
+  const before = rules
+    .slice(0, index)
+    .filter((rule) => rule.enabled && !ruleStops(rule))
+    .reduce((n, rule) => n + ruleForwards(rule), 0);
+  const after = worstCaseForwards(rules.slice(replaces ? index + 1 : index));
+  return { before, after };
+}
+
+/**
+ * The rules in the order the script runs them: forwarding from the out of
+ * office card goes ahead of every rule, and ends the script unless it keeps
+ * a copy here.
+ */
+export function inRunOrder(rules: RuleForwards[], forward: VacationForward | null | undefined): RuleForwards[] {
+  if (!forward?.enabled) return rules;
+  return [{ enabled: true, actions: [{ type: 'forward', value: forward.to }], stopProcessing: !forward.keepCopy }, ...rules];
+}
+
+// ── App additions: what the rule editor and the rule list show ──
+
+type ListedRule = RuleForwards & Pick<FilterRule, 'id' | 'origin'>;
+
+/**
+ * forwardsAround for the rule the editor has open in the filter list: the
+ * edited rule where it is, a new one below Bulwark's own rules (where
+ * addRule puts it), behind the out of office forwarding.
+ */
+export function forwardsOfEditedRule(
+  rules: ListedRule[],
+  forward: VacationForward | null | undefined,
+  editedRuleId: string | undefined,
+): { before: number; after: number } {
+  const runOrder = inRunOrder(rules, forward);
+  const editedIndex = editedRuleId ? rules.findIndex((r) => r.id === editedRuleId) : -1;
+  const own = rules.filter((r) => r.origin !== 'external' && r.origin !== 'opaque').length;
+  return forwardsAround(runOrder, runOrder.length - rules.length + (editedIndex >= 0 ? editedIndex : own), editedIndex >= 0);
+}
+
+export interface RuleForwardLimit {
+  /** The server's cap on forwards per message, when it has one above 0. */
+  limit: number | null;
+  /** One message can collect more forwards than the cap with this rule. */
+  overLimit: boolean;
+  /** The server runs no forward at all (a cap of 0). */
+  unavailable: boolean;
+  /** Whether another action may become a forward. */
+  canAddForward: boolean;
+}
+
+/** What the rule editor says about the server's redirect cap for the rule as edited. */
+export function ruleForwardLimit(
+  rule: Pick<FilterRule, 'actions' | 'stopProcessing'>,
+  maxRedirects: number | null | undefined,
+  around: { before: number; after: number },
+): RuleForwardLimit {
+  const count = ruleForwards(rule);
+  const others = around.before + (ruleStops(rule) ? 0 : around.after);
+  const limit = typeof maxRedirects === 'number' && maxRedirects > 0 ? maxRedirects : null;
+  const unavailable = maxRedirects === 0;
+  return {
+    limit,
+    overLimit: limit !== null && count + others > limit,
+    unavailable,
+    canAddForward: !unavailable && (limit === null || count + others < limit),
+  };
+}
+
+/** What the rule list says: the rules' forwards over the cap, or ignored by a server without forwarding. */
+export function rulesForwardLimit(
+  rules: RuleForwards[],
+  forward: VacationForward | null | undefined,
+  maxRedirects: number | null | undefined,
+): { tooMany: boolean; ignored: boolean } {
+  return {
+    tooMany: typeof maxRedirects === 'number' && maxRedirects > 0 && worstCaseForwards(inRunOrder(rules, forward)) > maxRedirects,
+    ignored: maxRedirects === 0 && worstCaseForwards(rules) > 0,
+  };
+}
diff --git a/src/lib/vacation-limits.ts b/src/lib/vacation-limits.ts
new file mode 100644
index 00000000..bd084126
--- /dev/null
+++ b/src/lib/vacation-limits.ts
@@ -0,0 +1,29 @@
+// Stalwart refuses a vacation subject of 512 bytes or more and a text or
+// HTML body of 2048 bytes or more, answering only "Field could not be set"
+// (webmail components/settings/vacation-settings.tsx).
+export const STALWART_VACATION_LIMITS = { subject: 511, body: 2047 } as const;
+
+export type VacationLimits = { subject: number; body: number };
+
+/** The UTF-8 length of `value` in bytes. */
+export function utf8Length(value: string): number {
+  let bytes = 0;
+  for (const ch of value) {
+    const code = ch.codePointAt(0)!;
+    bytes += code < 0x80 ? 1 : code < 0x800 ? 2 : code < 0x10000 ? 3 : 4;
+  }
+  return bytes;
+}
+
+/** Which parts of the reply as a save sends it are over the server's limits; none without limits. */
+export function vacationOversize(
+  reply: { subject: string; textBody: string; htmlBody: string | null },
+  limits: VacationLimits | null,
+): { subject: boolean; body: boolean } {
+  if (!limits) return { subject: false, body: false };
+  return {
+    subject: utf8Length(reply.subject) > limits.subject,
+    body: utf8Length(reply.textBody) > limits.body
+      || (reply.htmlBody !== null && utf8Length(reply.htmlBody) > limits.body),
+  };
+}
diff --git a/src/stores/__tests__/filter-store.test.ts b/src/stores/__tests__/filter-store.test.ts
index 89109e43..019ec910 100644
--- a/src/stores/__tests__/filter-store.test.ts
+++ b/src/stores/__tests__/filter-store.test.ts
@@ -346,6 +346,17 @@ describe('readVacationFilters', () => {
     });
   });
 
+  it('counts the most forwards the filter rules give one message, for the limit of the forwarding', async () => {
+    api.getSieveCapabilities.mockReturnValue({ ...WITH_INCLUDE, sieveExtensions: STALWART_EXTENSIONS });
+    api.getSieveScripts.mockResolvedValue([{ id: 's1', name: 'filters', blobId: 'b1', isActive: true }]);
+    const forward = { type: 'forward' as const, value: 'x@example.org' };
+    api.getSieveScriptContent.mockResolvedValue(generateScript([
+      makeRule({ id: 'a', actions: [forward] }),
+      makeRule({ id: 'b', actions: [forward] }),
+    ]));
+    expect((await readVacationFilters()).otherForwards).toBe(2);
+  });
+
   it('sets up forwarding in the vacation period, creating the script when there is none', async () => {
     api.getSieveCapabilities.mockReturnValue({ ...WITH_INCLUDE, sieveExtensions: STALWART_EXTENSIONS });
     api.getSieveScripts.mockResolvedValue([{ id: 'v1', name: 'vacation', blobId: 'bv', isActive: true }]);
diff --git a/src/stores/__tests__/vacation-store.test.ts b/src/stores/__tests__/vacation-store.test.ts
index 42149389..00230cdb 100644
--- a/src/stores/__tests__/vacation-store.test.ts
+++ b/src/stores/__tests__/vacation-store.test.ts
@@ -18,7 +18,7 @@ vi.mock('../../api/sieve', () => ({ isSieveSupported: vi.fn(() => false) }));
 
 const NO_FILTERS = {
   includesVacation: false, forward: null, forwardAvailable: false,
-  audience: null, audienceAvailable: false, notRunning: false,
+  audience: null, audienceAvailable: false, notRunning: false, otherForwards: 0,
 };
 
 vi.mock('../filter-store', () => ({
diff --git a/src/stores/filter-store.ts b/src/stores/filter-store.ts
index 09237068..57bd2f43 100644
--- a/src/stores/filter-store.ts
+++ b/src/stores/filter-store.ts
@@ -7,6 +7,7 @@ import { generateScript, supportsSpamGuard, VACATION_SCRIPT_NAME } from '../lib/
 import { supportsPeriods } from '../lib/sieve/period';
 import { isValidVacationForward, withVacationPeriod } from '../lib/sieve/vacation-forward';
 import { isValidVacationAudience, normalizeVacationAudience } from '../lib/sieve/vacation-audience';
+import { worstCaseForwards } from '../lib/sieve/forward-limit';
 import {
   createSieveScript,
   getSieveAccountId,
@@ -344,6 +345,12 @@ export interface VacationFilters {
    * client), or no script runs at all. Saving the card sets it right.
    */
   notRunning: boolean;
+  /**
+   * The most forwards the filter rules let one message collect (see
+   * worstCaseForwards). Forwarding that keeps a copy runs ahead of them, so
+   * it shares the server's redirect limit with these.
+   */
+  otherForwards: number;
 }
 
 /** See VacationFilters.notRunning. */
@@ -381,6 +388,7 @@ export async function readVacationFilters(accountId?: string): Promise<VacationF
     // Told apart by the envelope sender, which the auto-reply goes to.
     audienceAvailable: filtersUsable && !!capabilities?.sieveExtensions?.includes('envelope'),
     notRunning: storedButIdle(!!target?.isActive, !!vacationScript?.isActive, forward, audience),
+    otherForwards: worstCaseForwards(parsed?.rules ?? []),
   };
 }
 
diff --git a/src/stores/vacation-store.ts b/src/stores/vacation-store.ts
index dddd01ae..a63432ac 100644
--- a/src/stores/vacation-store.ts
+++ b/src/stores/vacation-store.ts
@@ -52,6 +52,8 @@ export interface VacationState {
   audienceAvailable: boolean;
   /** Stored forwarding or recipients do not run until the card is saved (see readVacationFilters). */
   notRunning: boolean;
+  /** The most forwards the filter rules let one message collect (see readVacationFilters). */
+  otherForwards: number;
   isLoading: boolean;
   isSaving: boolean;
   error: string | null;
@@ -80,6 +82,7 @@ const NO_FILTERS = {
   audience: null,
   audienceAvailable: false,
   notRunning: false,
+  otherForwards: 0,
 };
 
 const INITIAL = {
@@ -106,6 +109,7 @@ function fromFilters(filters: VacationFilters) {
     audience: filters.audience,
     audienceAvailable: filters.audienceAvailable,
     notRunning: filters.notRunning,
+    otherForwards: filters.otherForwards,
   };
 }