diff --git a/core/modules/update/tests/src/Functional/UpdateSettingsFormTest.php b/core/modules/update/tests/src/Functional/UpdateSettingsFormTest.php
index f653d35dce..ba04807a0d 100644
--- a/core/modules/update/tests/src/Functional/UpdateSettingsFormTest.php
+++ b/core/modules/update/tests/src/Functional/UpdateSettingsFormTest.php
@@ -46,22 +46,43 @@ public function testUpdateSettingsForm() {
     $this->drupalGet($url);
     $this->assertSession()->fieldExists('update_notify_emails');
 
+    $values_to_enter = [
+      'http://example.com',
+      'sofie@example.com',
+      'http://example.com/also-not-an-email-address',
+      'dries@example.com',
+    ];
+
     // Fill in `http://example.com` as the email address to notify. We expect
-    // this to trigger a validation error, because it's not an email address.
+    // this to trigger a validation error, because it's not an email address,
+    // and for the corresponding form item to be highlighted.
+    $this->assertSession()->fieldExists('update_notify_emails')->setValue($values_to_enter[0]);
     $this->submitForm([], 'Save configuration');
     $this->assertSession()->statusMessageNotExists(MessengerInterface::TYPE_STATUS);
     $this->assertSession()->statusMessageNotExists(MessengerInterface::TYPE_WARNING);
     $this->assertSession()->statusMessageContains('http://example.com is not a valid email address.', MessengerInterface::TYPE_ERROR);
+    $this->assertTrue($this->assertSession()->fieldExists('update_notify_emails')->hasClass('error'));
+    $this->assertSame([], $this->config('update.settings')->get('notification.emails'));
+
+    // Next, set multiple invalid email addresses, and assert the same as above
+    // except the message should be adjusted now.
+    $this->assertSession()->fieldExists('update_notify_emails')->setValue(implode("\n", $values_to_enter));
+    $this->submitForm([], 'Save configuration');
+    $this->assertSession()->statusMessageNotExists(MessengerInterface::TYPE_STATUS);
+    $this->assertSession()->statusMessageNotExists(MessengerInterface::TYPE_WARNING);
+    $this->assertSession()->statusMessageContains('http://example.com, http://example.com/also-not-an-email-address are not valid email addresses.', MessengerInterface::TYPE_ERROR);
+    $this->assertTrue($this->assertSession()->fieldExists('update_notify_emails')->hasClass('error'));
     $this->assertSame([], $this->config('update.settings')->get('notification.emails'));
 
-    // Now fill in a valid email address, now the form should be saved
+    // Now fill in valid email addresses, now the form should be saved
     // successfully.
-    $this->assertSession()->fieldExists('update_notify_emails')->setValue('sofie@example.com');
+    $this->assertSession()->fieldExists('update_notify_emails')->setValue("$values_to_enter[1]\r\n$values_to_enter[3]");
     $this->submitForm([], 'Save configuration');
     $this->assertSession()->statusMessageContains('The configuration options have been saved.', MessengerInterface::TYPE_STATUS);
     $this->assertSession()->statusMessageNotExists(MessengerInterface::TYPE_WARNING);
     $this->assertSession()->statusMessageNotExists(MessengerInterface::TYPE_ERROR);
-    $this->assertSame(['sofie@example.com'], $this->config('update.settings')->get('notification.emails'));
+    $this->assertFalse($this->assertSession()->fieldExists('update_notify_emails')->hasClass('error'));
+    $this->assertSame(['sofie@example.com', 'dries@example.com'], $this->config('update.settings')->get('notification.emails'));
   }
 
 }