From c555973b6b722db4e1b6dd4863ab30d9787b84eb Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Fri, 1 May 2026 01:42:06 +0200
Subject: [PATCH] feat: configurable sub-addressing delimiter #239

---
 components/email/email-composer.tsx        |  9 +--
 components/email/email-identity-badge.tsx  | 10 +--
 components/identity/sub-address-helper.tsx |  6 +-
 components/settings/composing-settings.tsx | 21 +++++-
 lib/__tests__/sub-addressing.test.ts       | 74 ++++++++++++++++++++++
 lib/sub-addressing.ts                      | 50 +++++++++++----
 locales/cs/common.json                     |  7 +-
 locales/de/common.json                     |  7 +-
 locales/en/common.json                     |  7 +-
 locales/es/common.json                     |  7 +-
 locales/fr/common.json                     |  7 +-
 locales/it/common.json                     |  7 +-
 locales/ja/common.json                     |  7 +-
 locales/ko/common.json                     |  7 +-
 locales/lv/common.json                     |  7 +-
 locales/nl/common.json                     |  7 +-
 locales/pl/common.json                     |  7 +-
 locales/pt/common.json                     |  7 +-
 locales/ru/common.json                     |  7 +-
 locales/tr/common.json                     |  7 +-
 locales/uk/common.json                     |  7 +-
 locales/zh/common.json                     |  7 +-
 stores/settings-store.ts                   | 11 ++++
 23 files changed, 252 insertions(+), 41 deletions(-)

diff --git a/components/email/email-composer.tsx b/components/email/email-composer.tsx
index 7cf32b07..350a182c 100644
--- a/components/email/email-composer.tsx
+++ b/components/email/email-composer.tsx
@@ -124,6 +124,7 @@ export function EmailComposer({
   const tCommon = useTranslations('common');
   const timeFormat = useSettingsStore((state) => state.timeFormat);
   const plainTextMode = useSettingsStore((state) => state.plainTextMode);
+  const subAddressDelimiter = useSettingsStore((state) => state.subAddressDelimiter);
   const autoSelectReplyIdentity = useSettingsStore((state) => state.autoSelectReplyIdentity);
   const attachmentReminderEnabled = useSettingsStore((state) => state.attachmentReminderEnabled);
   const attachmentReminderKeywords = useSettingsStore((state) => state.attachmentReminderKeywords);
@@ -729,7 +730,7 @@ export function EmailComposer({
     // Generate sub-addressed email if tag is set
     const fromEmail = currentIdentity?.email
       ? subAddressTag
-        ? generateSubAddress(currentIdentity.email, subAddressTag)
+        ? generateSubAddress(currentIdentity.email, subAddressTag, subAddressDelimiter)
         : currentIdentity.email
       : undefined;
 
@@ -906,7 +907,7 @@ export function EmailComposer({
 
     const fromEmail = currentIdentity?.email
       ? subAddressTag
-        ? generateSubAddress(currentIdentity.email, subAddressTag)
+        ? generateSubAddress(currentIdentity.email, subAddressTag, subAddressDelimiter)
         : currentIdentity.email
       : undefined;
 
@@ -1215,7 +1216,7 @@ export function EmailComposer({
                 >
                   {identities.map((identity) => {
                     const displayEmail = subAddressTag
-                      ? generateSubAddress(identity.email, subAddressTag)
+                      ? generateSubAddress(identity.email, subAddressTag, subAddressDelimiter)
                       : identity.email;
                     return (
                       <option key={identity.id} value={identity.id}>
@@ -1228,7 +1229,7 @@ export function EmailComposer({
                 <span className="text-sm text-foreground flex-1 truncate">
                   {subAddressTag ? (
                     <span className="font-mono">
-                      {generateSubAddress(primaryIdentity?.email || '', subAddressTag)}
+                      {generateSubAddress(primaryIdentity?.email || '', subAddressTag, subAddressDelimiter)}
                     </span>
                   ) : (
                     <>
diff --git a/components/email/email-identity-badge.tsx b/components/email/email-identity-badge.tsx
index d320d2ea..ce6ed3e6 100644
--- a/components/email/email-identity-badge.tsx
+++ b/components/email/email-identity-badge.tsx
@@ -5,6 +5,7 @@ import { Mail, Tag } from 'lucide-react';
 import { cn } from '@/lib/utils';
 import type { Email, Identity } from '@/lib/jmap/types';
 import { parseSubAddress } from '@/lib/sub-addressing';
+import { useSettingsStore } from '@/stores/settings-store';
 
 interface EmailIdentityBadgeProps {
   email: Email;
@@ -20,12 +21,13 @@ export function EmailIdentityBadge({
   className,
 }: EmailIdentityBadgeProps) {
   const t = useTranslations('identities.badge');
+  const subAddressDelimiter = useSettingsStore((state) => state.subAddressDelimiter);
 
   const fromAddress = email.from?.[0]?.email;
   if (!fromAddress) return null;
 
   // Parse the from address to check for sub-addressing
-  const parsedFrom = parseSubAddress(fromAddress);
+  const parsedFrom = parseSubAddress(fromAddress, subAddressDelimiter);
 
   // Find matching identity (email sent BY the user)
   const matchingIdentity = identities.find(
@@ -37,7 +39,7 @@ export function EmailIdentityBadge({
   if (!matchingIdentity) {
     // Check all TO addresses for sub-address tags matching user's identities
     for (const recipient of email.to || []) {
-      const parsedTo = parseSubAddress(recipient.email);
+      const parsedTo = parseSubAddress(recipient.email, subAddressDelimiter);
       if (parsedTo.tag) {
         // Check if this base email matches any of the user's identities
         const matchingToIdentity = identities.find(
@@ -70,7 +72,7 @@ export function EmailIdentityBadge({
           title={t('sub_address_tag', { tag: displayTag })}
         >
           <Tag className="w-3 h-3" />
-          <span className="font-mono">+{displayTag}</span>
+          <span className="font-mono">{subAddressDelimiter}{displayTag}</span>
         </div>
       );
     }
@@ -114,7 +116,7 @@ export function EmailIdentityBadge({
           aria-label={t('sub_address_tag', { tag: displayTag })}
         >
           <Tag className="w-3 h-3" />
-          <span className="font-mono">{t('subaddress_tag', { tag: displayTag })}</span>
+          <span className="font-mono">{subAddressDelimiter}{displayTag}</span>
         </div>
       )}
 
diff --git a/components/identity/sub-address-helper.tsx b/components/identity/sub-address-helper.tsx
index 7d8f43b8..83c86069 100644
--- a/components/identity/sub-address-helper.tsx
+++ b/components/identity/sub-address-helper.tsx
@@ -7,6 +7,7 @@ import { cn } from '@/lib/utils';
 import { Button } from '@/components/ui/button';
 import { Input } from '@/components/ui/input';
 import { useIdentityStore } from '@/stores/identity-store';
+import { useSettingsStore } from '@/stores/settings-store';
 import {
   generateSubAddress,
   extractDomain,
@@ -35,6 +36,7 @@ export function SubAddressHelper({
   const popoverRef = useRef<HTMLDivElement>(null);
 
   const { subAddress, addRecentTag, addTagSuggestion } = useIdentityStore();
+  const subAddressDelimiter = useSettingsStore((state) => state.subAddressDelimiter);
 
   // Get suggestions based on recipient (memoized for performance)
   const suggestions = useMemo(() => {
@@ -47,7 +49,7 @@ export function SubAddressHelper({
   }, [recipientEmails]);
 
   // Generate preview
-  const preview = tag ? generateSubAddress(baseEmail, tag) : baseEmail;
+  const preview = tag ? generateSubAddress(baseEmail, tag, subAddressDelimiter) : baseEmail;
 
   // Close popover when clicking outside
   useEffect(() => {
@@ -226,7 +228,7 @@ export function SubAddressHelper({
 
           {/* Help Text */}
           <div className="mb-3 text-xs text-muted-foreground">
-            {t('help_text')}
+            {t('help_text', { delimiter: subAddressDelimiter })}
           </div>
 
           {/* Use Address Button */}
diff --git a/components/settings/composing-settings.tsx b/components/settings/composing-settings.tsx
index abcebb89..1f8dbc13 100644
--- a/components/settings/composing-settings.tsx
+++ b/components/settings/composing-settings.tsx
@@ -4,8 +4,12 @@ import { useState, useCallback } from 'react';
 import { useTranslations } from 'next-intl';
 import { useConfig } from '@/hooks/use-config';
 import { useSettingsStore } from '@/stores/settings-store';
-import { SettingsSection, SettingItem, ToggleSwitch } from './settings-section';
+import { SettingsSection, SettingItem, Select, ToggleSwitch } from './settings-section';
 import { Mail, X } from 'lucide-react';
+import {
+  SUPPORTED_SUB_ADDRESS_DELIMITERS,
+  type SubAddressDelimiter,
+} from '@/lib/sub-addressing';
 
 export function ComposingSettings() {
   const t = useTranslations('settings.email_behavior');
@@ -17,6 +21,7 @@ export function ComposingSettings() {
     autoSelectReplyIdentity,
     attachmentReminderEnabled,
     attachmentReminderKeywords,
+    subAddressDelimiter,
     updateSetting,
   } = useSettingsStore();
 
@@ -40,6 +45,20 @@ export function ComposingSettings() {
         />
       </SettingItem>
 
+      <SettingItem
+        label={t('sub_address_delimiter.label')}
+        description={t('sub_address_delimiter.description', { delimiter: subAddressDelimiter })}
+      >
+        <Select
+          value={subAddressDelimiter}
+          onChange={(value) => updateSetting('subAddressDelimiter', value as SubAddressDelimiter)}
+          options={SUPPORTED_SUB_ADDRESS_DELIMITERS.map((delim) => ({
+            value: delim,
+            label: t('sub_address_delimiter.option', { delimiter: delim }),
+          }))}
+        />
+      </SettingItem>
+
       <SettingItem label={t('attachment_reminder.label')} description={t('attachment_reminder.description')}>
         <ToggleSwitch
           checked={attachmentReminderEnabled}
diff --git a/lib/__tests__/sub-addressing.test.ts b/lib/__tests__/sub-addressing.test.ts
index b97092fb..00bb9400 100644
--- a/lib/__tests__/sub-addressing.test.ts
+++ b/lib/__tests__/sub-addressing.test.ts
@@ -6,6 +6,9 @@ import {
   suggestTagsForDomain,
   isValidTag,
   getTagValidationError,
+  isSupportedSubAddressDelimiter,
+  SUPPORTED_SUB_ADDRESS_DELIMITERS,
+  DEFAULT_SUB_ADDRESS_DELIMITER,
   MAX_TAG_LENGTH,
 } from '../sub-addressing';
 
@@ -354,3 +357,74 @@ describe('getTagValidationError', () => {
     expect(getTagValidationError('日本語')).toBe('INVALID_CHARS');
   });
 });
+
+describe('custom delimiter', () => {
+  describe('parseSubAddress with non-default delimiter', () => {
+    it('should parse with "-" delimiter', () => {
+      const result = parseSubAddress('user-shopping@example.com', '-');
+      expect(result.baseUser).toBe('user');
+      expect(result.tag).toBe('shopping');
+    });
+
+    it('should parse with "." delimiter', () => {
+      const result = parseSubAddress('user.shopping@example.com', '.');
+      expect(result.baseUser).toBe('user');
+      expect(result.tag).toBe('shopping');
+    });
+
+    it('should parse with "=" delimiter', () => {
+      const result = parseSubAddress('user=shopping@example.com', '=');
+      expect(result.baseUser).toBe('user');
+      expect(result.tag).toBe('shopping');
+    });
+
+    it('should ignore "+" when "-" is configured as the delimiter', () => {
+      const result = parseSubAddress('user+shopping@example.com', '-');
+      expect(result.baseUser).toBe('user+shopping');
+      expect(result.tag).toBeNull();
+    });
+
+    it('should split on first occurrence when delimiter appears multiple times', () => {
+      const result = parseSubAddress('alice-shop-orders@example.com', '-');
+      expect(result.baseUser).toBe('alice');
+      expect(result.tag).toBe('shop-orders');
+    });
+  });
+
+  describe('generateSubAddress with non-default delimiter', () => {
+    it('should generate using "-" delimiter', () => {
+      expect(generateSubAddress('user@example.com', 'shopping', '-')).toBe('user-shopping@example.com');
+    });
+
+    it('should generate using "." delimiter', () => {
+      expect(generateSubAddress('user@example.com', 'shopping', '.')).toBe('user.shopping@example.com');
+    });
+
+    it('should replace existing tag using the configured delimiter', () => {
+      expect(generateSubAddress('user-old@example.com', 'new', '-')).toBe('user-new@example.com');
+    });
+
+    it('should not strip a "+" sign in the local part when delimiter is "-"', () => {
+      // "+" is not the delimiter so it should remain part of the base user
+      expect(generateSubAddress('user+plus@example.com', 'tag', '-')).toBe('user+plus-tag@example.com');
+    });
+  });
+
+  describe('isSupportedSubAddressDelimiter', () => {
+    it('accepts every supported delimiter', () => {
+      for (const delim of SUPPORTED_SUB_ADDRESS_DELIMITERS) {
+        expect(isSupportedSubAddressDelimiter(delim)).toBe(true);
+      }
+    });
+
+    it('rejects unsupported characters', () => {
+      expect(isSupportedSubAddressDelimiter('_')).toBe(false);
+      expect(isSupportedSubAddressDelimiter('++')).toBe(false);
+      expect(isSupportedSubAddressDelimiter('')).toBe(false);
+    });
+
+    it('default delimiter is supported', () => {
+      expect(isSupportedSubAddressDelimiter(DEFAULT_SUB_ADDRESS_DELIMITER)).toBe(true);
+    });
+  });
+});
diff --git a/lib/sub-addressing.ts b/lib/sub-addressing.ts
index 41425a5f..e4530858 100644
--- a/lib/sub-addressing.ts
+++ b/lib/sub-addressing.ts
@@ -1,12 +1,23 @@
 /**
- * Sub-addressing utilities for user+tag@domain.com format
+ * Sub-addressing utilities for user{delimiter}tag@domain.com format
  * Works server-side automatically - no JMAP API calls needed
+ *
+ * The delimiter character is configurable per server (RFC 5233). Common
+ * choices: "+" (Postfix, Stalwart default), "-" (qmail), ".", "=".
  */
 
 // Constants for tag validation
 const MAX_TAG_LENGTH = 30;
 const TAG_REGEX = /^[a-zA-Z0-9-]{1,30}$/;
 
+export const DEFAULT_SUB_ADDRESS_DELIMITER = '+';
+export const SUPPORTED_SUB_ADDRESS_DELIMITERS = ['+', '-', '.', '='] as const;
+export type SubAddressDelimiter = (typeof SUPPORTED_SUB_ADDRESS_DELIMITERS)[number];
+
+export function isSupportedSubAddressDelimiter(value: string): value is SubAddressDelimiter {
+  return (SUPPORTED_SUB_ADDRESS_DELIMITERS as readonly string[]).includes(value);
+}
+
 export type TagValidationErrorCode =
   | 'EMPTY'
   | 'TOO_LONG'
@@ -22,10 +33,14 @@ export interface ParsedAddress {
 }
 
 /**
- * Parse an email address to extract sub-address tag
- * Example: "user+shopping@example.com" -> { baseUser: "user", tag: "shopping" }
+ * Parse an email address to extract sub-address tag.
+ * The first occurrence of the delimiter in the local part separates the
+ * base user from the tag, matching the behavior of Postfix/qmail/Sieve.
  */
-export function parseSubAddress(email: string): ParsedAddress {
+export function parseSubAddress(
+  email: string,
+  delimiter: string = DEFAULT_SUB_ADDRESS_DELIMITER,
+): ParsedAddress {
   const [localPart, domain] = email.split('@');
 
   if (!localPart || !domain) {
@@ -38,9 +53,9 @@ export function parseSubAddress(email: string): ParsedAddress {
     };
   }
 
-  const plusIndex = localPart.indexOf('+');
+  const delimiterIndex = localPart.indexOf(delimiter);
 
-  if (plusIndex === -1) {
+  if (delimiterIndex === -1) {
     return {
       localPart,
       baseUser: localPart,
@@ -50,8 +65,8 @@ export function parseSubAddress(email: string): ParsedAddress {
     };
   }
 
-  const baseUser = localPart.substring(0, plusIndex);
-  const tag = localPart.substring(plusIndex + 1);
+  const baseUser = localPart.substring(0, delimiterIndex);
+  const tag = localPart.substring(delimiterIndex + delimiter.length);
 
   return {
     localPart,
@@ -63,18 +78,25 @@ export function parseSubAddress(email: string): ParsedAddress {
 }
 
 /**
- * Generate a sub-addressed email
- * Example: generateSubAddress("user@example.com", "shopping") -> "user+shopping@example.com"
+ * Generate a sub-addressed email.
+ * Example: generateSubAddress("user@example.com", "shopping", "+") -> "user+shopping@example.com"
  */
-export function generateSubAddress(baseEmail: string, tag: string): string {
+export function generateSubAddress(
+  baseEmail: string,
+  tag: string,
+  delimiter: string = DEFAULT_SUB_ADDRESS_DELIMITER,
+): string {
   const [localPart, domain] = baseEmail.split('@');
 
   if (!localPart || !domain || !tag) {
     return baseEmail;
   }
 
-  // Remove existing tag if present
-  const cleanLocal = localPart.split('+')[0];
+  // Strip an existing tag if one is already present
+  const existingDelimiterIndex = localPart.indexOf(delimiter);
+  const cleanLocal = existingDelimiterIndex === -1
+    ? localPart
+    : localPart.substring(0, existingDelimiterIndex);
 
   // Sanitize tag (alphanumeric and dash only)
   const cleanTag = tag.replace(/[^a-zA-Z0-9-]/g, '').toLowerCase();
@@ -83,7 +105,7 @@ export function generateSubAddress(baseEmail: string, tag: string): string {
     return baseEmail;
   }
 
-  return `${cleanLocal}+${cleanTag}@${domain}`;
+  return `${cleanLocal}${delimiter}${cleanTag}@${domain}`;
 }
 
 /**
diff --git a/locales/cs/common.json b/locales/cs/common.json
index 042527de..39cace66 100644
--- a/locales/cs/common.json
+++ b/locales/cs/common.json
@@ -911,6 +911,11 @@
         "label": "Automaticky vybírat adresu pro odpověď",
         "description": "Při odpovídání automaticky přepnout adresu odesílatele na identitu, která původně obdržela zprávu"
       },
+      "sub_address_delimiter": {
+        "label": "Oddělovač sub-adresy",
+        "description": "Znak oddělující uživatelské jméno od sub-adresy. Zvolte oddělovač používaný vaším poštovním serverem (např. uzivatel{delimiter}stitek@domena.cz).",
+        "option": "{delimiter}  (uzivatel{delimiter}stitek@domena.cz)"
+      },
       "attachment_click_action": {
         "label": "Akce po kliknutí na přílohu",
         "description": "Vyberte, zda má kliknutí na přílohu zobrazit náhled, nebo ji ihned stáhnout",
@@ -1756,7 +1761,7 @@
       "use_address": "Použít tuto adresu",
       "invalid_tag": "Štítek může obsahovat pouze písmena, číslice a pomlčky",
       "tag_too_long": "Štítek může mít maximálně 30 znaků",
-      "help_text": "Zprávy odeslané na adresu uzivatel+stitek@domena.cz budou doručeny do vaší doručené pošty",
+      "help_text": "Zprávy odeslané na adresu uzivatel{delimiter}stitek@domena.cz budou doručeny do vaší doručené pošty",
       "validation": {
         "empty": "Štítek nesmí být prázdný",
         "too_long": "Štítek může mít maximálně {max} znaků",
diff --git a/locales/de/common.json b/locales/de/common.json
index 6bf3fe55..84f1c896 100644
--- a/locales/de/common.json
+++ b/locales/de/common.json
@@ -911,6 +911,11 @@
         "label": "Antwortadresse automatisch wählen",
         "description": "Beim Antworten die Absenderadresse automatisch auf die Identität umstellen, die die ursprüngliche Nachricht erhalten hat"
       },
+      "sub_address_delimiter": {
+        "label": "Sub-Adress-Trennzeichen",
+        "description": "Zeichen, das Ihren Benutzernamen vom Sub-Adress-Tag trennt. Verwenden Sie das von Ihrem Mailserver verwendete Trennzeichen (z. B. benutzer{delimiter}tag@domain.de).",
+        "option": "{delimiter}  (benutzer{delimiter}tag@domain.de)"
+      },
       "attachment_click_action": {
         "label": "Aktion beim Klick auf Anhänge",
         "description": "Festlegen, ob ein Dateianhang beim Anklicken in der Vorschau geöffnet oder sofort heruntergeladen wird",
@@ -1756,7 +1761,7 @@
       "use_address": "Diese Adresse verwenden",
       "invalid_tag": "Tag darf nur alphanumerisch und Bindestriche enthalten",
       "tag_too_long": "Tag darf maximal 30 Zeichen lang sein",
-      "help_text": "E-Mails an benutzer+tag@domain.de werden in Ihrem Posteingang ankommen",
+      "help_text": "E-Mails an benutzer{delimiter}tag@domain.de werden in Ihrem Posteingang ankommen",
       "validation": {
         "empty": "Tag darf nicht leer sein",
         "too_long": "Tag darf maximal {max} Zeichen lang sein",
diff --git a/locales/en/common.json b/locales/en/common.json
index 97551606..255c2a77 100644
--- a/locales/en/common.json
+++ b/locales/en/common.json
@@ -911,6 +911,11 @@
         "label": "Auto-select Reply Address",
         "description": "When replying, automatically switch the From address to the identity that originally received the message"
       },
+      "sub_address_delimiter": {
+        "label": "Sub-Address Delimiter",
+        "description": "Character separating your username from a sub-address tag. Match the delimiter your mail server uses (e.g. user{delimiter}tag@domain.com).",
+        "option": "{delimiter}  (user{delimiter}tag@domain.com)"
+      },
       "attachment_click_action": {
         "label": "Attachment Click Action",
         "description": "Choose whether clicking a file attachment previews it or downloads it immediately",
@@ -1756,7 +1761,7 @@
       "use_address": "Use This Address",
       "invalid_tag": "Tag must be alphanumeric and dashes only",
       "tag_too_long": "Tag must be 30 characters or less",
-      "help_text": "Emails sent to user+tag@domain.com will arrive in your inbox",
+      "help_text": "Emails sent to user{delimiter}tag@domain.com will arrive in your inbox",
       "validation": {
         "empty": "Tag cannot be empty",
         "too_long": "Tag must be {max} characters or less",
diff --git a/locales/es/common.json b/locales/es/common.json
index 9ba0be0f..8b5ab5ee 100644
--- a/locales/es/common.json
+++ b/locales/es/common.json
@@ -906,6 +906,11 @@
         "label": "Seleccionar dirección de respuesta automáticamente",
         "description": "Al responder, cambia automáticamente la dirección del remitente a la identidad que recibió el mensaje original"
       },
+      "sub_address_delimiter": {
+        "label": "Delimitador de sub-dirección",
+        "description": "Carácter que separa tu nombre de usuario de la etiqueta de sub-dirección. Usa el delimitador que utilice tu servidor de correo (por ejemplo, usuario{delimiter}etiqueta@dominio.com).",
+        "option": "{delimiter}  (usuario{delimiter}etiqueta@dominio.com)"
+      },
       "show_preview": {
         "label": "Mostrar Vista Previa",
         "description": "Mostrar vista previa del correo en la lista",
@@ -1756,7 +1761,7 @@
       "use_address": "Usar Esta Dirección",
       "invalid_tag": "La etiqueta debe ser solo alfanumérica y guiones",
       "tag_too_long": "La etiqueta debe tener 30 caracteres o menos",
-      "help_text": "Los correos enviados a usuario+etiqueta@dominio.com llegarán a su bandeja de entrada",
+      "help_text": "Los correos enviados a usuario{delimiter}etiqueta@dominio.com llegarán a su bandeja de entrada",
       "validation": {
         "empty": "La etiqueta no puede estar vacía",
         "too_long": "La etiqueta debe tener {max} caracteres o menos",
diff --git a/locales/fr/common.json b/locales/fr/common.json
index db327696..3cb79da7 100644
--- a/locales/fr/common.json
+++ b/locales/fr/common.json
@@ -906,6 +906,11 @@
         "label": "Sélection automatique de l'adresse de réponse",
         "description": "Lors d'une réponse, bascule automatiquement l'adresse d'expédition vers l'identité qui a reçu le message d'origine"
       },
+      "sub_address_delimiter": {
+        "label": "Délimiteur de sous-adresse",
+        "description": "Caractère séparant votre nom d'utilisateur de l'étiquette de sous-adresse. Utilisez le délimiteur configuré sur votre serveur de messagerie (par ex. utilisateur{delimiter}tag@domaine.com).",
+        "option": "{delimiter}  (utilisateur{delimiter}tag@domaine.com)"
+      },
       "show_preview": {
         "label": "Afficher l'aperçu",
         "description": "Afficher l'aperçu de l'email dans la liste",
@@ -1756,7 +1761,7 @@
       "use_address": "Utiliser cette adresse",
       "invalid_tag": "Le tag doit contenir uniquement des lettres, chiffres et tirets",
       "tag_too_long": "Le tag doit faire 30 caractères ou moins",
-      "help_text": "Les emails envoyés à utilisateur+tag@domaine.com arriveront dans votre boîte de réception",
+      "help_text": "Les emails envoyés à utilisateur{delimiter}tag@domaine.com arriveront dans votre boîte de réception",
       "validation": {
         "empty": "Le tag ne peut pas être vide",
         "too_long": "Le tag doit faire {max} caractères ou moins",
diff --git a/locales/it/common.json b/locales/it/common.json
index 4ae4db2d..8bb58cc1 100644
--- a/locales/it/common.json
+++ b/locales/it/common.json
@@ -906,6 +906,11 @@
         "label": "Seleziona automaticamente l'indirizzo di risposta",
         "description": "Quando rispondi, passa automaticamente l'indirizzo mittente all'identità che ha ricevuto il messaggio originale"
       },
+      "sub_address_delimiter": {
+        "label": "Delimitatore sub-indirizzo",
+        "description": "Carattere che separa il tuo nome utente dall'etichetta del sub-indirizzo. Usa il delimitatore configurato sul tuo server di posta (es. utente{delimiter}tag@dominio.com).",
+        "option": "{delimiter}  (utente{delimiter}tag@dominio.com)"
+      },
       "show_preview": {
         "label": "Mostra anteprima testo",
         "description": "Visualizza l'anteprima del messaggio nell'elenco",
@@ -1756,7 +1761,7 @@
       "use_address": "Usa questo indirizzo",
       "invalid_tag": "Il tag deve contenere solo caratteri alfanumerici e trattini",
       "tag_too_long": "Il tag deve essere di massimo 30 caratteri",
-      "help_text": "I messaggi inviati a utente+tag@dominio.com arriveranno nella tua casella di posta",
+      "help_text": "I messaggi inviati a utente{delimiter}tag@dominio.com arriveranno nella tua casella di posta",
       "validation": {
         "empty": "Il tag non può essere vuoto",
         "too_long": "Il tag deve essere di massimo {max} caratteri",
diff --git a/locales/ja/common.json b/locales/ja/common.json
index 1d2ee0e6..bafcde8e 100644
--- a/locales/ja/common.json
+++ b/locales/ja/common.json
@@ -906,6 +906,11 @@
         "label": "返信元アドレスを自動選択",
         "description": "返信時に、元のメッセージを受信したIDへ差出人アドレスを自動的に切り替えます"
       },
+      "sub_address_delimiter": {
+        "label": "サブアドレス区切り文字",
+        "description": "ユーザー名とサブアドレスタグを区切る文字です。お使いのメールサーバーが使用する区切り文字に合わせてください（例: user{delimiter}tag@domain.com）。",
+        "option": "{delimiter}  (user{delimiter}tag@domain.com)"
+      },
       "show_preview": {
         "label": "プレビューテキストを表示",
         "description": "リストにメールのプレビューを表示",
@@ -1756,7 +1761,7 @@
       "use_address": "このアドレスを使用",
       "invalid_tag": "タグは英数字とハイフンのみ使用できます",
       "tag_too_long": "タグは30文字以内にしてください",
-      "help_text": "user+tag@domain.comに送信されたメールは受信トレイに届きます",
+      "help_text": "user{delimiter}tag@domain.comに送信されたメールは受信トレイに届きます",
       "validation": {
         "empty": "タグは空にできません",
         "too_long": "タグは{max}文字以内にしてください",
diff --git a/locales/ko/common.json b/locales/ko/common.json
index 8020071e..a0e39d38 100644
--- a/locales/ko/common.json
+++ b/locales/ko/common.json
@@ -911,6 +911,11 @@
         "label": "답장 시 보내는 사람 자동 선택",
         "description": "답장할 때 메일을 받았던 주소로 보내는 사람을 자동으로 변경해요"
       },
+      "sub_address_delimiter": {
+        "label": "서브 주소 구분자",
+        "description": "사용자 이름과 서브 주소 태그를 나누는 문자예요. 메일 서버가 사용하는 구분자에 맞춰 주세요 (예: user{delimiter}tag@domain.com).",
+        "option": "{delimiter}  (user{delimiter}tag@domain.com)"
+      },
       "attachment_click_action": {
         "label": "첨부파일 클릭 동작",
         "description": "파일을 클릭했을 때 미리보기를 할지, 바로 다운로드할지 선택해 주세요",
@@ -1756,7 +1761,7 @@
       "use_address": "이 주소 사용하기",
       "invalid_tag": "태그는 알파벳, 숫자, 대시(-)만 쓸 수 있어요",
       "tag_too_long": "태그는 30자 이하여야 해요",
-      "help_text": "user+tag@domain.com 으로 보낸 메일은 내 받은편지함으로 들어와요",
+      "help_text": "user{delimiter}tag@domain.com 으로 보낸 메일은 내 받은편지함으로 들어와요",
       "validation": {
         "empty": "태그를 비워둘 수 없어요",
         "too_long": "태그는 {max}자 이하여야 해요",
diff --git a/locales/lv/common.json b/locales/lv/common.json
index 105c8af8..1520839a 100644
--- a/locales/lv/common.json
+++ b/locales/lv/common.json
@@ -906,6 +906,11 @@
         "label": "Automātiski izvēlēties atbildes adresi",
         "description": "Atbildot automātiski izmantot to kontu, uz kuru vēstule tika saņemta"
       },
+      "sub_address_delimiter": {
+        "label": "Apakšadreses atdalītājs",
+        "description": "Zīme, kas atdala lietotājvārdu no apakšadreses tagu. Izvēlieties atdalītāju, ko lieto jūsu pasta serveris (piem. lietotajs{delimiter}tags@domens.lv).",
+        "option": "{delimiter}  (lietotajs{delimiter}tags@domens.lv)"
+      },
       "show_preview": {
         "label": "Rādīt priekšskatījuma tekstu",
         "description": "Rādīt vēstules fragmentu sarakstā",
@@ -1756,7 +1761,7 @@
       "use_address": "Izmantot šo adresi",
       "invalid_tag": "Tags var saturēt tikai burtus, ciparus un domuzīmes",
       "tag_too_long": "Tags nedrīkst pārsniegt 30 rakstzīmes",
-      "help_text": "Vēstules uz lietotajs+tags@domens.lv nonāks jūsu pastkastē",
+      "help_text": "Vēstules uz lietotajs{delimiter}tags@domens.lv nonāks jūsu pastkastē",
       "validation": {
         "empty": "Tags nevar būt tukšs",
         "too_long": "Tags nedrīkst pārsniegt {max} rakstzīmes",
diff --git a/locales/nl/common.json b/locales/nl/common.json
index 7785e383..d6420a3c 100644
--- a/locales/nl/common.json
+++ b/locales/nl/common.json
@@ -906,6 +906,11 @@
         "label": "Antwoordadres automatisch selecteren",
         "description": "Schakel bij het beantwoorden automatisch het Van-adres om naar de identiteit die het oorspronkelijke bericht ontving"
       },
+      "sub_address_delimiter": {
+        "label": "Sub-adres scheidingsteken",
+        "description": "Teken dat je gebruikersnaam scheidt van het sub-adres-label. Gebruik het scheidingsteken dat je mailserver gebruikt (bv. gebruiker{delimiter}tag@domein.nl).",
+        "option": "{delimiter}  (gebruiker{delimiter}tag@domein.nl)"
+      },
       "show_preview": {
         "label": "Voorbeeldtekst tonen",
         "description": "E-mailvoorbeeld weergeven in de lijst",
@@ -1756,7 +1761,7 @@
       "use_address": "Dit adres gebruiken",
       "invalid_tag": "Tag mag alleen letters, cijfers en streepjes bevatten",
       "tag_too_long": "Tag mag maximaal 30 tekens bevatten",
-      "help_text": "E-mails verzonden naar gebruiker+tag@domein.nl komen in je postvak IN aan",
+      "help_text": "E-mails verzonden naar gebruiker{delimiter}tag@domein.nl komen in je postvak IN aan",
       "validation": {
         "empty": "Tag mag niet leeg zijn",
         "too_long": "Tag mag maximaal {max} tekens bevatten",
diff --git a/locales/pl/common.json b/locales/pl/common.json
index 14e52484..d66d0508 100644
--- a/locales/pl/common.json
+++ b/locales/pl/common.json
@@ -911,6 +911,11 @@
         "label": "Automatycznie wybieraj adres odpowiedzi",
         "description": "Podczas odpowiadania automatycznie przełączaj adres nadawcy na tożsamość, która pierwotnie otrzymała wiadomość"
       },
+      "sub_address_delimiter": {
+        "label": "Separator sub-adresu",
+        "description": "Znak oddzielający Twoją nazwę użytkownika od tagu sub-adresu. Użyj separatora zgodnego z Twoim serwerem pocztowym (np. user{delimiter}tag@domain.com).",
+        "option": "{delimiter}  (user{delimiter}tag@domain.com)"
+      },
       "attachment_click_action": {
         "label": "Akcja po kliknięciu załącznika",
         "description": "Wybierz, czy kliknięcie załącznika pliku ma pokazać podgląd, czy od razu go pobrać",
@@ -1756,7 +1761,7 @@
       "use_address": "Użyj tego adresu",
       "invalid_tag": "Tag może zawierać tylko litery, cyfry i myślniki",
       "tag_too_long": "Tag może mieć maksymalnie 30 znaków",
-      "help_text": "Wiadomości wysłane na adres user+tag@domain.com trafią do Twojej skrzynki odbiorczej",
+      "help_text": "Wiadomości wysłane na adres user{delimiter}tag@domain.com trafią do Twojej skrzynki odbiorczej",
       "validation": {
         "empty": "Tag nie może być pusty",
         "too_long": "Tag może mieć maksymalnie {max} znaków",
diff --git a/locales/pt/common.json b/locales/pt/common.json
index 62a7229b..d0ed8c0f 100644
--- a/locales/pt/common.json
+++ b/locales/pt/common.json
@@ -906,6 +906,11 @@
         "label": "Selecionar automaticamente o endereço de resposta",
         "description": "Ao responder, muda automaticamente o endereço do remetente para a identidade que recebeu a mensagem original"
       },
+      "sub_address_delimiter": {
+        "label": "Delimitador de sub-endereço",
+        "description": "Caractere que separa seu nome de usuário da tag de sub-endereço. Use o delimitador configurado no seu servidor de e-mail (ex.: usuario{delimiter}tag@dominio.com).",
+        "option": "{delimiter}  (usuario{delimiter}tag@dominio.com)"
+      },
       "show_preview": {
         "label": "Mostrar Texto de Visualização",
         "description": "Exibir visualização do e-mail na lista",
@@ -1756,7 +1761,7 @@
       "use_address": "Usar Este Endereço",
       "invalid_tag": "A tag deve conter apenas caracteres alfanuméricos e hífens",
       "tag_too_long": "A tag deve ter no máximo 30 caracteres",
-      "help_text": "E-mails enviados para usuario+tag@dominio.com chegarão na sua caixa de entrada",
+      "help_text": "E-mails enviados para usuario{delimiter}tag@dominio.com chegarão na sua caixa de entrada",
       "validation": {
         "empty": "A tag não pode estar vazia",
         "too_long": "A tag deve ter no máximo {max} caracteres",
diff --git a/locales/ru/common.json b/locales/ru/common.json
index 3d52bbcc..e6cac299 100644
--- a/locales/ru/common.json
+++ b/locales/ru/common.json
@@ -906,6 +906,11 @@
         "label": "Автоматически выбирать адрес для ответа",
         "description": "При ответе автоматически переключать адрес отправителя на ту учетную запись, которая получила исходное сообщение"
       },
+      "sub_address_delimiter": {
+        "label": "Разделитель суб-адресов",
+        "description": "Символ, отделяющий имя пользователя от тега суб-адреса. Используйте разделитель, настроенный на вашем почтовом сервере (например, user{delimiter}tag@domain.com).",
+        "option": "{delimiter}  (user{delimiter}tag@domain.com)"
+      },
       "show_preview": {
         "label": "Показывать текст предпросмотра",
         "description": "Отображать предпросмотр письма в списке",
@@ -1756,7 +1761,7 @@
       "use_address": "Использовать этот адрес",
       "invalid_tag": "Тег должен содержать только буквы, цифры и дефисы",
       "tag_too_long": "Тег не должен превышать 30 символов",
-      "help_text": "Письма на адрес user+tag@domain.com будут приходить в ваш ящик",
+      "help_text": "Письма на адрес user{delimiter}tag@domain.com будут приходить в ваш ящик",
       "validation": {
         "empty": "Тег не может быть пустым",
         "too_long": "Тег не должен превышать {max} символов",
diff --git a/locales/tr/common.json b/locales/tr/common.json
index 228659a4..8d6dd4f7 100644
--- a/locales/tr/common.json
+++ b/locales/tr/common.json
@@ -911,6 +911,11 @@
         "label": "Yanıt Adresini Otomatik Seç",
         "description": "Yanıtlarken, Kimden adresini iletiyi başlangıçta alan kimliğe otomatik olarak değiştir"
       },
+      "sub_address_delimiter": {
+        "label": "Alt Adres Ayırıcı",
+        "description": "Kullanıcı adını alt adres etiketinden ayıran karakter. Posta sunucunuzun kullandığı ayırıcıyı seçin (ör. kullanici{delimiter}etiket@domain.com).",
+        "option": "{delimiter}  (kullanici{delimiter}etiket@domain.com)"
+      },
       "attachment_click_action": {
         "label": "Ek Tıklama İşlemi",
         "description": "Bir dosya ekine tıklandığında önizleme mi yoksa anında indirme mi yapılacağını seçin",
@@ -1756,7 +1761,7 @@
       "use_address": "Bu Adresi Kullan",
       "invalid_tag": "Etiket yalnızca harf, rakam ve tire içermelidir",
       "tag_too_long": "Etiket en fazla 30 karakter olmalıdır",
-      "help_text": "kullanici+etiket@domain.com adresine gönderilen e-postalar gelen kutunuza ulaşır",
+      "help_text": "kullanici{delimiter}etiket@domain.com adresine gönderilen e-postalar gelen kutunuza ulaşır",
       "validation": {
         "empty": "Etiket boş olamaz",
         "too_long": "Etiket en fazla {max} karakter olmalıdır",
diff --git a/locales/uk/common.json b/locales/uk/common.json
index f154b2f0..61fc82f0 100644
--- a/locales/uk/common.json
+++ b/locales/uk/common.json
@@ -911,6 +911,11 @@
         "label": "Автоматичний вибір адреси для відповіді",
         "description": "Під час відповіді автоматично змінюйте адресу відправника на особу, яка спочатку отримала повідомлення"
       },
+      "sub_address_delimiter": {
+        "label": "Розділювач під-адреси",
+        "description": "Символ, який відокремлює ім'я користувача від мітки під-адреси. Використовуйте розділювач, налаштований на вашому поштовому сервері (напр. user{delimiter}tag@domain.com).",
+        "option": "{delimiter}  (user{delimiter}tag@domain.com)"
+      },
       "attachment_click_action": {
         "label": "Вкладення Натисніть Дія",
         "description": "Виберіть, чи клацання вкладеного файлу попередньо переглядає його чи негайно завантажує",
@@ -1756,7 +1761,7 @@
       "use_address": "Використовуйте цю адресу",
       "invalid_tag": "Тег має бути лише буквено-цифровим і тире",
       "tag_too_long": "Тег має містити 30 символів або менше",
-      "help_text": "Електронні листи, надіслані на user+tag@domain.com, надходитимуть до вашої скриньки",
+      "help_text": "Електронні листи, надіслані на user{delimiter}tag@domain.com, надходитимуть до вашої скриньки",
       "validation": {
         "empty": "Тег не може бути порожнім",
         "too_long": "Тег має містити не більше {max} символів",
diff --git a/locales/zh/common.json b/locales/zh/common.json
index fb42829e..a73466e2 100644
--- a/locales/zh/common.json
+++ b/locales/zh/common.json
@@ -911,6 +911,11 @@
         "label": "自动选择回复地址",
         "description": "回复时自动将发件人地址切换为最初收到该邮件的身份"
       },
+      "sub_address_delimiter": {
+        "label": "子地址分隔符",
+        "description": "用于分隔用户名和子地址标签的字符。请选择与您的邮件服务器一致的分隔符（例如 user{delimiter}tag@domain.com）。",
+        "option": "{delimiter}  (user{delimiter}tag@domain.com)"
+      },
       "attachment_click_action": {
         "label": "附件单击操作",
         "description": "选择点击附件时是预览还是直接下载",
@@ -1756,7 +1761,7 @@
       "use_address": "使用此地址",
       "invalid_tag": "标签只能是字母数字和破折号",
       "tag_too_long": "标签不得超过 30 个字符",
-      "help_text": "发送到 user+tag@domain.com 的邮件将送达您的收件箱",
+      "help_text": "发送到 user{delimiter}tag@domain.com 的邮件将送达您的收件箱",
       "validation": {
         "empty": "标签不能为空",
         "too_long": "标签不得超过 {max} 个字符",
diff --git a/stores/settings-store.ts b/stores/settings-store.ts
index 9d315c71..479961e0 100644
--- a/stores/settings-store.ts
+++ b/stores/settings-store.ts
@@ -4,6 +4,11 @@ import { useThemeStore } from './theme-store';
 import { useLocaleStore } from './locale-store';
 import type { NotificationSoundChoice } from '@/lib/notification-sound';
 import { apiFetch } from '@/lib/browser-navigation';
+import {
+  DEFAULT_SUB_ADDRESS_DELIMITER,
+  isSupportedSubAddressDelimiter,
+  type SubAddressDelimiter,
+} from '@/lib/sub-addressing';
 
 // Use console directly to avoid circular dependency with lib/debug.ts
 // (debug.ts imports useSettingsStore for debugMode check)
@@ -138,6 +143,7 @@ interface SettingsState {
   defaultReplyMode: ReplyMode;
   autoSelectReplyIdentity: boolean;
   plainTextMode: boolean; // Send plain text only (no rich text editor)
+  subAddressDelimiter: SubAddressDelimiter; // Character separating user from tag (e.g. "user+tag@")
 
   // Privacy & Security
   sessionTimeout: number; // minutes (0 = never)
@@ -286,6 +292,7 @@ const DEFAULT_SETTINGS = {
   defaultReplyMode: 'reply' as ReplyMode,
   autoSelectReplyIdentity: false,
   plainTextMode: false,
+  subAddressDelimiter: DEFAULT_SUB_ADDRESS_DELIMITER as SubAddressDelimiter,
 
   // Privacy & Security
   sessionTimeout: 0, // Never
@@ -456,6 +463,7 @@ export const useSettingsStore = create<SettingsState>()(
           defaultReplyMode: state.defaultReplyMode,
           autoSelectReplyIdentity: state.autoSelectReplyIdentity,
           plainTextMode: state.plainTextMode,
+          subAddressDelimiter: state.subAddressDelimiter,
           sessionTimeout: state.sessionTimeout,
           emailNotificationsEnabled: state.emailNotificationsEnabled,
           emailNotificationSound: state.emailNotificationSound,
@@ -508,6 +516,9 @@ export const useSettingsStore = create<SettingsState>()(
           // Apply settings
           Object.keys(settings).forEach((key) => {
             if (key in DEFAULT_SETTINGS) {
+              if (key === 'subAddressDelimiter' && !isSupportedSubAddressDelimiter(settings[key])) {
+                return;
+              }
               set({ [key]: settings[key] });
             }
           });