diff --git a/migration/1781015951873-NormalizeUserDataMailLowercase.js b/migration/1781015951873-NormalizeUserDataMailLowercase.js
new file mode 100644
index 0000000000..91cfd25d7f
--- /dev/null
+++ b/migration/1781015951873-NormalizeUserDataMailLowercase.js
@@ -0,0 +1,38 @@
+// Normalize all stored e-mail addresses to lowercase and add a functional index on LOWER(mail).
+//
+// Background: input lowercase-normalization (Util.toLowerCaseTrim) only arrived with PR #2695
+// (2025-12-23), so accounts created earlier hold mixed-case mails. After the MSSQL->PostgreSQL
+// cutover (PR #3620, 2026-05-22) the duplicate-detection lookup `getUsersByMail` (exact `mail = ?`)
+// became case-sensitive and stopped matching those mixed-case rows, allowing duplicate accounts
+// for the same address (e.g. `Samuel.kullmann@...` vs `samuel.kullmann@...`).
+//
+// This migration lowercases the legacy data so it is consistent with the now case-insensitive
+// lookup. The index is intentionally NON-unique here: case-collision duplicates still exist and
+// must be resolved via the merge campaign before the UNIQUE variant can be created (separate PR).
+//
+// Note: lowercasing is lossy (original casing is not recoverable), so `down` is a no-op for the
+// data updates and only drops the index.
+
+/**
+ * @typedef {import('typeorm').MigrationInterface} MigrationInterface
+ * @typedef {import('typeorm').QueryRunner} QueryRunner
+ */
+
+module.exports = class NormalizeUserDataMailLowercase1781015951873 {
+  name = 'NormalizeUserDataMailLowercase1781015951873';
+
+  async up(queryRunner) {
+    await queryRunner.query(
+      `UPDATE "user_data" SET "mail" = LOWER("mail") WHERE "mail" IS NOT NULL AND "mail" <> LOWER("mail")`,
+    );
+    await queryRunner.query(
+      `UPDATE "recommendation" SET "recommendedMail" = LOWER("recommendedMail") WHERE "recommendedMail" IS NOT NULL AND "recommendedMail" <> LOWER("recommendedMail")`,
+    );
+    await queryRunner.query(`CREATE INDEX "IDX_user_data_mail_lower" ON "user_data" (LOWER("mail"))`);
+  }
+
+  async down(queryRunner) {
+    // Data lowercasing is irreversible; only the index is dropped.
+    await queryRunner.query(`DROP INDEX "public"."IDX_user_data_mail_lower"`);
+  }
+};
diff --git a/src/subdomains/generic/user/models/user-data/__tests__/user-data.service.spec.ts b/src/subdomains/generic/user/models/user-data/__tests__/user-data.service.spec.ts
index ad7edb0767..1618d3b153 100644
--- a/src/subdomains/generic/user/models/user-data/__tests__/user-data.service.spec.ts
+++ b/src/subdomains/generic/user/models/user-data/__tests__/user-data.service.spec.ts
@@ -1,5 +1,7 @@
 import { createMock } from '@golevelup/ts-jest';
 import { Test, TestingModule } from '@nestjs/testing';
+import { ConflictException } from '@nestjs/common';
+import { FindOperator } from 'typeorm';
 import { RepositoryFactory } from 'src/shared/repositories/repository.factory';
 import { CountryService } from 'src/shared/models/country/country.service';
 import { FiatService } from 'src/shared/models/fiat/fiat.service';
@@ -65,6 +67,44 @@ describe('UserDataService', () => {
     service = module.get(UserDataService);
   });
 
+  describe('getUsersByMail', () => {
+    it('matches case-insensitively via LOWER(mail) and passes a lowercased parameter', async () => {
+      userDataRepo.find.mockResolvedValue([]);
+
+      await service.getUsersByMail('Samuel.Kullmann@Startmail.com');
+
+      const where = userDataRepo.find.mock.calls[0][0].where as { mail: FindOperator<string> };
+      expect(where.mail).toBeInstanceOf(FindOperator);
+      expect(where.mail.type).toBe('raw');
+      // `getSql` is a getter returning the SQL generator; invoke it with the column alias
+      expect(where.mail.getSql?.('"UserData"."mail"')).toBe('LOWER("UserData"."mail") = :mail');
+      expect(where.mail.objectLiteralParameters).toEqual({ mail: 'samuel.kullmann@startmail.com' });
+    });
+
+    it('omits the status filter when onlyValidUser is false', async () => {
+      userDataRepo.find.mockResolvedValue([]);
+
+      await service.getUsersByMail('a@b.com', false);
+
+      const where = userDataRepo.find.mock.calls[0][0].where as { status?: unknown };
+      expect(where.status).toBeUndefined();
+    });
+  });
+
+  describe('checkMail', () => {
+    it('throws a conflict when another account already uses the same mail (case-insensitive)', async () => {
+      const userData = Object.assign(new UserData(), { id: 1, kycType: 'DFX' });
+      const conflictUser = Object.assign(new UserData(), { id: 2, kycType: 'DFX' });
+
+      // getUsersByMail() resolves to the case-variant conflicting account
+      userDataRepo.find.mockResolvedValue([conflictUser]);
+
+      await expect(service.checkMail(userData, 'samuel.kullmann@startmail.com')).rejects.toBeInstanceOf(
+        ConflictException,
+      );
+    });
+  });
+
   describe('updateUserData', () => {
     it('does not pass kycSteps or users to save() to prevent stale-collection FK clobber', async () => {
       const fakeKycSteps = [{ id: 10 }] as UserData['kycSteps'];
diff --git a/src/subdomains/generic/user/models/user-data/user-data.service.ts b/src/subdomains/generic/user/models/user-data/user-data.service.ts
index 8035b831ee..8b6d2c3716 100644
--- a/src/subdomains/generic/user/models/user-data/user-data.service.ts
+++ b/src/subdomains/generic/user/models/user-data/user-data.service.ts
@@ -47,7 +47,7 @@ import { TfaLevel, TfaService } from 'src/subdomains/generic/kyc/services/tfa.se
 import { MailContext } from 'src/subdomains/supporting/notification/enums';
 import { SpecialExternalAccountService } from 'src/subdomains/supporting/payment/services/special-external-account.service';
 import { TransactionService } from 'src/subdomains/supporting/payment/services/transaction.service';
-import { Equal, FindOptionsRelations, In, IsNull, MoreThan, Not } from 'typeorm';
+import { Equal, FindOptionsRelations, In, IsNull, MoreThan, Not, Raw } from 'typeorm';
 import { WebhookService } from '../../services/webhook/webhook.service';
 import { MergeReason } from '../account-merge/account-merge.entity';
 import { AccountMergeService } from '../account-merge/account-merge.service';
@@ -196,7 +196,7 @@ export class UserDataService {
   ): Promise<UserData[]> {
     return this.userDataRepo.find({
       where: {
-        mail,
+        mail: Raw((alias) => `LOWER(${alias}) = :mail`, { mail: mail?.toLowerCase() }),
         status: onlyValidUser
           ? In([UserDataStatus.ACTIVE, UserDataStatus.NA, UserDataStatus.KYC_ONLY, UserDataStatus.DEACTIVATED])
           : undefined,