diff --git a/examples/oidc/boot.ts b/examples/oidc/boot.ts
index 51d7f3edf..f820181d1 100644
--- a/examples/oidc/boot.ts
+++ b/examples/oidc/boot.ts
@@ -50,7 +50,7 @@ try {
          *   return false;
          * }
          */
-        validateEmail: async () => true,
+        validateEmail: async () => ({ valid: true as const }),
       },
     },
     adminUiConfig: {
diff --git a/packages/api/src/errors.ts b/packages/api/src/errors.ts
index a445630fe..1ff32c0f5 100644
--- a/packages/api/src/errors.ts
+++ b/packages/api/src/errors.ts
@@ -241,14 +241,24 @@ export const CyclicProductBundlingNotSupportedError = createError(
   'Cyclic bundling detected, make sure bundled product is not the same as the bundle product itself',
 );
 
+export const EmailFormatInvalidError = createError(
+  'EmailFormatInvalid',
+  'Email address format is invalid',
+);
+
 export const EmailAlreadyExistsError = createError(
   'EmailAlreadyExists',
-  'Email already exists or is invalid',
+  'Email address already exists',
+);
+
+export const UsernameTooShortError = createError(
+  'UsernameTooShort',
+  'Username must be at least 3 characters',
 );
 
 export const UsernameAlreadyExistsError = createError(
   'UsernameAlreadyExists',
-  'Username already exists or is invalid',
+  'Username already exists',
 );
 
 export const UsernameOrEmailRequiredError = createError(
diff --git a/packages/core-users/src/module/configureUsersModule.ts b/packages/core-users/src/module/configureUsersModule.ts
index 3438c5847..644c9f38f 100644
--- a/packages/core-users/src/module/configureUsersModule.ts
+++ b/packages/core-users/src/module/configureUsersModule.ts
@@ -324,8 +324,9 @@ export const configureUsersModule = async (moduleInput: ModuleInput<UserSettings
       const services: Record<string, any> = {};
 
       if (email) {
-        if (!(await userSettings.validateEmail(email))) {
-          throw new Error(`E-Mail address ${email} is invalid`, { cause: 'EMAIL_INVALID' });
+        const emailResult = await userSettings.validateEmail(email);
+        if (!emailResult.valid) {
+          throw new Error(`E-Mail address ${email} is invalid`, { cause: emailResult.reason });
         }
       }
 
@@ -353,8 +354,9 @@ export const configureUsersModule = async (moduleInput: ModuleInput<UserSettings
       };
 
       if (username) {
-        if (!(await userSettings.validateUsername(username))) {
-          throw new Error(`Username ${username} is invalid`, { cause: 'USERNAME_INVALID' });
+        const usernameResult = await userSettings.validateUsername(username);
+        if (!usernameResult.valid) {
+          throw new Error(`Username ${username} is invalid`, { cause: usernameResult.reason });
         }
         doc.username = username;
       }
@@ -411,8 +413,9 @@ export const configureUsersModule = async (moduleInput: ModuleInput<UserSettings
     },
 
     async addEmail(userId: string, address: string): Promise<void> {
-      if (!(await userSettings.validateEmail(address))) {
-        throw new Error(`E-Mail address ${address} is invalid`, { cause: 'EMAIL_INVALID' });
+      const emailResult = await userSettings.validateEmail(address);
+      if (!emailResult.valid) {
+        throw new Error(`E-Mail address ${address} is invalid`, { cause: emailResult.reason });
       }
       await this.updateUser(
         { _id: userId, 'emails.address': { $not: insensitiveTrimmedRegexOperator(address) } },
@@ -686,8 +689,9 @@ export const configureUsersModule = async (moduleInput: ModuleInput<UserSettings
     },
 
     async setUsername(userId: string, username: string) {
-      if (!(await userSettings.validateUsername(username))) {
-        throw new Error(`Username ${username} is invalid`, { cause: 'USERNAME_INVALID' });
+      const usernameResult = await userSettings.validateUsername(username);
+      if (!usernameResult.valid) {
+        throw new Error(`Username ${username} is invalid`, { cause: usernameResult.reason });
       }
       const user = await Users.findOneAndUpdate(
         { _id: userId },
diff --git a/packages/core-users/src/users-settings.test.ts b/packages/core-users/src/users-settings.test.ts
new file mode 100644
index 000000000..69ef215b1
--- /dev/null
+++ b/packages/core-users/src/users-settings.test.ts
@@ -0,0 +1,83 @@
+import { describe, it, before } from 'node:test';
+import assert from 'node:assert';
+import { userSettings } from './users-settings.ts';
+
+const makeDb = (count: number) =>
+  ({
+    collection: () => ({
+      countDocuments: async () => count,
+    }),
+  }) as any;
+
+describe('defaultValidateEmail', () => {
+  describe('format validation', () => {
+    before(() => {
+      userSettings.configureSettings({}, makeDb(0));
+    });
+
+    it('returns EMAIL_FORMAT_INVALID when email has no @', async () => {
+      const result = await userSettings.validateEmail('notanemail');
+      assert.deepStrictEqual(result, { valid: false, reason: 'EMAIL_FORMAT_INVALID' });
+    });
+
+    it('returns EMAIL_FORMAT_INVALID for empty string', async () => {
+      const result = await userSettings.validateEmail('');
+      assert.deepStrictEqual(result, { valid: false, reason: 'EMAIL_FORMAT_INVALID' });
+    });
+
+    it('returns valid: true for a correctly formatted new email', async () => {
+      const result = await userSettings.validateEmail('new@example.com');
+      assert.deepStrictEqual(result, { valid: true });
+    });
+  });
+
+  describe('duplicate detection', () => {
+    before(() => {
+      userSettings.configureSettings({}, makeDb(1));
+    });
+
+    it('returns EMAIL_ALREADY_EXISTS when email is taken', async () => {
+      const result = await userSettings.validateEmail('existing@example.com');
+      assert.deepStrictEqual(result, { valid: false, reason: 'EMAIL_ALREADY_EXISTS' });
+    });
+  });
+});
+
+describe('defaultValidateUsername', () => {
+  describe('length validation', () => {
+    before(() => {
+      userSettings.configureSettings({}, makeDb(0));
+    });
+
+    it('returns USERNAME_TOO_SHORT for a 2-character username', async () => {
+      const result = await userSettings.validateUsername('ab');
+      assert.deepStrictEqual(result, { valid: false, reason: 'USERNAME_TOO_SHORT' });
+    });
+
+    it('returns USERNAME_TOO_SHORT for empty string', async () => {
+      const result = await userSettings.validateUsername('');
+      assert.deepStrictEqual(result, { valid: false, reason: 'USERNAME_TOO_SHORT' });
+    });
+
+    it('returns valid: true for a username of exactly 3 characters', async () => {
+      const result = await userSettings.validateUsername('abc');
+      assert.deepStrictEqual(result, { valid: true });
+    });
+
+    it('returns valid: true for a longer valid username', async () => {
+      const result = await userSettings.validateUsername('newuser');
+      assert.deepStrictEqual(result, { valid: true });
+    });
+  });
+
+  describe('duplicate detection', () => {
+    before(() => {
+      userSettings.configureSettings({}, makeDb(1));
+    });
+
+    it('returns USERNAME_ALREADY_EXISTS when username is taken', async () => {
+      const result = await userSettings.validateUsername('existinguser');
+      assert.deepStrictEqual(result, { valid: false, reason: 'USERNAME_ALREADY_EXISTS' });
+    });
+  });
+});
diff --git a/packages/core-users/src/users-settings.ts b/packages/core-users/src/users-settings.ts
index 29926a7cf..86262f45b 100644
--- a/packages/core-users/src/users-settings.ts
+++ b/packages/core-users/src/users-settings.ts
@@ -15,20 +15,25 @@ export const UserAccountAction = {
 } as const;
 
 export type UserAccountAction = (typeof UserAccountAction)[keyof typeof UserAccountAction];
+export type ValidationResult = { valid: true } | { valid: false; reason: string };
+
 export interface UserSettings {
   mergeUserCartsOnLogin: boolean;
   autoMessagingAfterUserCreation: boolean;
   earliestValidTokenDate: (
     type: typeof UserAccountAction.VERIFY_EMAIL | typeof UserAccountAction.RESET_PASSWORD,
   ) => Date;
-  validateEmail: (email: string) => Promise<boolean>;
-  validateUsername: (username: string) => Promise<boolean>;
+  validateEmail: (email: string) => Promise<ValidationResult>;
+  validateUsername: (username: string) => Promise<ValidationResult>;
   validateNewUser: (user: UserRegistrationData) => Promise<UserRegistrationData>;
   validatePassword: (password: string) => Promise<boolean>;
   configureSettings: (options: UserSettingsOptions, db: mongodb.Db) => void;
 }
 
-export type UserSettingsOptions = Omit<Partial<UserSettings>, 'configureSettings'>;
+export type UserSettingsOptions = Omit<Partial<UserSettings>, 'configureSettings' | 'validateEmail' | 'validateUsername'> & {
+  validateEmail?: (email: string) => Promise<ValidationResult>;
+  validateUsername?: (username: string) => Promise<ValidationResult>;
+};
 
 const defaultAutoMessagingAfterUserCreation = true;
 const defaultMergeUserCartsOnLogin = true;
@@ -56,8 +61,8 @@ export const userSettings: UserSettings = {
   mergeUserCartsOnLogin: defaultMergeUserCartsOnLogin,
   earliestValidTokenDate: defaultEarliestValidTokenDate,
   validateNewUser: defaultValidateNewUser,
-  validateEmail: () => Promise.resolve(true),
-  validateUsername: () => Promise.resolve(true),
+  validateEmail: () => Promise.resolve({ valid: true } as ValidationResult),
+  validateUsername: () => Promise.resolve({ valid: true } as ValidationResult),
   validatePassword: () => Promise.resolve(true),
 
   configureSettings: (
@@ -72,21 +77,21 @@ export const userSettings: UserSettings = {
     },
     db: mongodb.Db,
   ) => {
-    const defaultValidateEmail = async (rawEmail: string) => {
-      if (!rawEmail?.includes?.('@')) return false;
+    const defaultValidateEmail = async (rawEmail: string): Promise<ValidationResult> => {
+      if (!rawEmail?.includes?.('@')) return { valid: false, reason: 'EMAIL_FORMAT_INVALID' };
       const emailAlreadyExists = await db
         .collection('users')
         .countDocuments({ 'emails.address': insensitiveTrimmedRegexOperator(rawEmail) }, { limit: 1 });
-      if (emailAlreadyExists) return false;
-      return true;
+      if (emailAlreadyExists) return { valid: false, reason: 'EMAIL_ALREADY_EXISTS' };
+      return { valid: true };
     };
-    const defaultValidateUsername = async (rawUsername: string) => {
-      if (rawUsername?.length < 3) return false;
+    const defaultValidateUsername = async (rawUsername: string): Promise<ValidationResult> => {
+      if (rawUsername?.length < 3) return { valid: false, reason: 'USERNAME_TOO_SHORT' };
       const usernameAlreadyExists = await db
         .collection('users')
         .countDocuments({ username: insensitiveTrimmedRegexOperator(rawUsername) }, { limit: 1 });
-      if (usernameAlreadyExists) return false;
-      return true;
+      if (usernameAlreadyExists) return { valid: false, reason: 'USERNAME_ALREADY_EXISTS' };
+      return { valid: true };
     };
 
     userSettings.mergeUserCartsOnLogin = mergeUserCartsOnLogin ?? defaultMergeUserCartsOnLogin;