diff --git a/core/modules/user/tests/src/Unit/Plugin/Validation/Constraint/UserMailRequiredValidatorTest.php b/core/modules/user/tests/src/Unit/Plugin/Validation/Constraint/UserMailRequiredValidatorTest.php
new file mode 100644
index 0000000000..c36ff97d73
--- /dev/null
+++ b/core/modules/user/tests/src/Unit/Plugin/Validation/Constraint/UserMailRequiredValidatorTest.php
@@ -0,0 +1,152 @@
+<?php
+
+namespace Drupal\Tests\user\Unit\Plugin\Validation\Constraint;
+
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Tests\UnitTestCase;
+use Drupal\user\Plugin\Validation\Constraint\UserMailRequired;
+use Drupal\user\Plugin\Validation\Constraint\UserMailRequiredValidator;
+use Drupal\user\UserInterface;
+use Drupal\user\UserStorageInterface;
+use Symfony\Component\Validator\Context\ExecutionContextInterface;
+
+/**
+ * @coversDefaultClass \Drupal\user\Plugin\Validation\Constraint\UserMailRequiredValidator
+ * @group user
+ */
+class UserMailRequiredValidatorTest extends UnitTestCase {
+
+  /**
+   * Creates a validator instance.
+   *
+   * @param bool $is_admin
+   *   Whether or not the current user is an administrator.
+   *
+   * @return \Drupal\user\Plugin\Validation\Constraint\UserMailRequiredValidator
+   *   The validator instance.
+   */
+  protected function createValidator($is_admin) {
+    // Setup mocks that don't need to change.
+    $unchanged_account = $this->prophesize(UserInterface::class);
+    $unchanged_account->getEmail()->willReturn(NULL);
+
+    $user_storage = $this->prophesize(UserStorageInterface::class);
+    $user_storage->loadUnchanged('current-user')->willReturn($unchanged_account->reveal());
+
+    $entity_type_manager = $this->prophesize(EntityTypeManagerInterface::class);
+    $entity_type_manager->getStorage('user')->willReturn($user_storage->reveal());
+
+    $current_user = $this->prophesize(AccountInterface::class);
+    $current_user->id()->willReturn('current-user');
+    $current_user->hasPermission("administer users")->willReturn($is_admin);
+    $container = new ContainerBuilder();
+    $container->set('entity_type.manager', $entity_type_manager->reveal());
+    $container->set('current_user', $current_user->reveal());
+    \Drupal::setContainer($container);
+    return new UserMailRequiredValidator();
+  }
+
+  /**
+   * @covers ::validate
+   *
+   * @dataProvider providerTestValidate
+   */
+  public function testValidate($items, $expected_violation, $is_admin = FALSE) {
+    $constraint = new UserMailRequired();
+
+    // If a violation is expected, then the context's addViolation method will
+    // be called, otherwise it should not be called.
+    $context = $this->prophesize(ExecutionContextInterface::class);
+
+    if ($expected_violation) {
+      $context->addViolation('@name field is required.', ["@name" => "label"])->shouldBeCalledTimes(1);
+    }
+    else {
+      $context->addViolation()->shouldNotBeCalled(1);
+    }
+
+    $validator = $this->createValidator($is_admin);
+    $validator->initialize($context->reveal());
+    $validator->validate($items, $constraint);
+  }
+
+  /**
+   * Data provider for ::testValidate().
+   */
+  public function providerTestValidate() {
+    $cases = [];
+
+    // Case 1: Empty user should be ignored.
+    $items = $this->prophesize(FieldItemListInterface::class);
+    $items->getEntity()->willReturn(NULL)->shouldBeCalledTimes(1);
+    $cases['Empty user should be ignored'] = [$items->reveal(), FALSE];
+
+    // Case 2: New users without an email should add a violation.
+    $items = $this->prophesize(FieldItemListInterface::class);
+    $account = $this->prophesize(UserInterface::class);
+    $account->isNew()->willReturn(TRUE);
+    $account->id()->shouldNotBeCalled();
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getLabel()->willReturn('label');
+    $account->getFieldDefinition("mail")->willReturn($field_definition->reveal())->shouldBeCalledTimes(1);
+    $items->getEntity()->willReturn($account->reveal())->shouldBeCalledTimes(1);
+    $items->isEmpty()->willReturn(TRUE);
+    $cases['New users without an email should add a violation'] = [$items->reveal(), TRUE];
+
+    // Case 3: Existing users without an email should add a violation.
+    $items = $this->prophesize(FieldItemListInterface::class);
+    $account = $this->prophesize(UserInterface::class);
+    $account->isNew()->willReturn(FALSE);
+    $account->id()->willReturn('current-user');
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getLabel()->willReturn('label');
+    $account->getFieldDefinition("mail")->willReturn($field_definition->reveal())->shouldBeCalledTimes(1);
+    $items->getEntity()->willReturn($account->reveal())->shouldBeCalledTimes(1);
+    $items->isEmpty()->willReturn(TRUE);
+    $cases['Existing users without an email should add a violation'] = [$items->reveal(), TRUE];
+
+    // Case 4: New users with an email should be ignored.
+    $items = $this->prophesize(FieldItemListInterface::class);
+    $account = $this->prophesize(UserInterface::class);
+    $account->isNew()->willReturn(TRUE);
+    $account->id()->shouldNotBeCalled();
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getLabel()->willReturn('label');
+    $account->getFieldDefinition("mail")->willReturn($field_definition->reveal())->shouldBeCalledTimes(1);
+    $items->getEntity()->willReturn($account->reveal())->shouldBeCalledTimes(1);
+    $items->isEmpty()->willReturn(FALSE);
+    $cases['New users with an email should be ignored'] = [$items->reveal(), FALSE];
+
+    // Case 5: Existing users with an email should be ignored.
+    $items = $this->prophesize(FieldItemListInterface::class);
+    $account = $this->prophesize(UserInterface::class);
+    $account->isNew()->willReturn(FALSE);
+    $account->id()->willReturn('current-user');
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getLabel()->willReturn('label');
+    $account->getFieldDefinition("mail")->willReturn($field_definition->reveal())->shouldBeCalledTimes(1);
+    $items->getEntity()->willReturn($account->reveal())->shouldBeCalledTimes(1);
+    $items->isEmpty()->willReturn(FALSE);
+    $cases['Existing users with an email should be ignored'] = [$items->reveal(), FALSE];
+
+    // Case 6: Existing users without an email should be ignored if the current
+    // user is an administrator.
+    $items = $this->prophesize(FieldItemListInterface::class);
+    $account = $this->prophesize(UserInterface::class);
+    $account->isNew()->willReturn(FALSE);
+    $account->id()->willReturn('current-user');
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getLabel()->willReturn('label');
+    $account->getFieldDefinition("mail")->willReturn($field_definition->reveal())->shouldBeCalledTimes(1);
+    $items->getEntity()->willReturn($account->reveal())->shouldBeCalledTimes(1);
+    $items->isEmpty()->willReturn(TRUE);
+    $cases['Existing users without an email should be ignored if the current user is an administrator.'] = [$items->reveal(), FALSE, TRUE];
+
+    return $cases;
+  }
+
+}