diff --git a/client/src/shared/lib/__tests__/is-synthetic-email.test.ts b/client/src/shared/lib/__tests__/is-synthetic-email.test.ts
new file mode 100644
index 0000000..9aba977
--- /dev/null
+++ b/client/src/shared/lib/__tests__/is-synthetic-email.test.ts
@@ -0,0 +1,24 @@
+import { describe, expect, it } from 'vitest'
+import { isSyntheticEmail } from '../is-synthetic-email'
+
+describe('isSyntheticEmail', () => {
+  it('returns true for vk.local domain', () => {
+    expect(isSyntheticEmail('vk_12345@vk.local')).toBe(true)
+  })
+
+  it('returns false for real email', () => {
+    expect(isSyntheticEmail('user@gmail.com')).toBe(false)
+  })
+
+  it('returns false for yandex email', () => {
+    expect(isSyntheticEmail('user@yandex.ru')).toBe(false)
+  })
+
+  it('returns false for empty string', () => {
+    expect(isSyntheticEmail('')).toBe(false)
+  })
+
+  it('returns false for email with vk.local as part of username', () => {
+    expect(isSyntheticEmail('vk.local@gmail.com')).toBe(false)
+  })
+})
diff --git a/client/src/shared/lib/is-synthetic-email.ts b/client/src/shared/lib/is-synthetic-email.ts
new file mode 100644
index 0000000..a3060de
--- /dev/null
+++ b/client/src/shared/lib/is-synthetic-email.ts
@@ -0,0 +1,5 @@
+const SYNTHETIC_DOMAINS = ['vk.local']
+
+export function isSyntheticEmail(email: string): boolean {
+  return SYNTHETIC_DOMAINS.some((domain) => email.endsWith(`@${domain}`))
+}