diff --git a/django/core/mail/backends/locmem.py b/django/core/mail/backends/locmem.py
index 642bfc49fb0..6826d09ee5f 100644
--- a/django/core/mail/backends/locmem.py
+++ b/django/core/mail/backends/locmem.py
@@ -20,5 +20,7 @@ class EmailBackend(BaseEmailBackend):
 
     def send_messages(self, messages):
         """Redirect messages to the dummy outbox"""
+        for message in messages:  # .message() triggers header validation
+            message.message()
         mail.outbox.extend(messages)
         return len(messages)
diff --git a/tests/regressiontests/mail/tests.py b/tests/regressiontests/mail/tests.py
index 3e9ae846502..33898cc1d54 100644
--- a/tests/regressiontests/mail/tests.py
+++ b/tests/regressiontests/mail/tests.py
@@ -498,6 +498,11 @@ class LocmemBackendTests(BaseEmailBackendTests, TestCase):
         connection2.send_messages([email])
         self.assertEqual(len(mail.outbox), 2)
 
+    def test_validate_multiline_headers(self):
+        # Ticket #18861 - Validate emails when using the locmem backend
+        with self.assertRaises(BadHeaderError):
+            send_mail('Subject\nMultiline', 'Content', 'from@example.com', ['to@example.com'])
+
 
 class FileBackendTests(BaseEmailBackendTests, TestCase):
     email_backend = 'django.core.mail.backends.filebased.EmailBackend'