[1.7.x] Fixed #23063 -- Convert \n and \r to \r\n when using the SMTP backend as per RFC.

Backport of 8d789449c7 from master.

tests/mail/tests.py

@@ -9,7 +9,7 @@ import smtpd
 import sys
 import tempfile
 import threading
-from smtplib import SMTPException
+from smtplib import SMTPException, SMTP
 from ssl import SSLError
 
 from django.core import mail
@@ -984,3 +984,37 @@ class SMTPBackendTests(BaseEmailBackendTests, SimpleTestCase):
         self.assertEqual(myemailbackend.timeout, 42)
         self.assertEqual(myemailbackend.connection.timeout, 42)
         myemailbackend.close()
+
+    def test_email_msg_uses_crlf(self):
+        """#23063 -- Test that RFC-compliant messages are sent over SMTP."""
+        send = SMTP.send
+        try:
+            smtp_messages = []
+
+            def mock_send(self, s):
+                smtp_messages.append(s)
+                return send(self, s)
+
+            SMTP.send = mock_send
+
+            email = EmailMessage('Subject', 'Content', 'from@example.com', ['to@example.com'])
+            mail.get_connection().send_messages([email])
+
+            # Find the actual message
+            msg = None
+            for i, m in enumerate(smtp_messages):
+                if m[:4] == 'data':
+                    msg = smtp_messages[i+1]
+                    break
+
+            self.assertTrue(msg)
+
+            if PY3:
+                msg = msg.decode('utf-8')
+            # Ensure that the message only contains CRLF and not combinations of CRLF, LF, and CR.
+            msg = msg.replace('\r\n', '')
+            self.assertNotIn('\r', msg)
+            self.assertNotIn('\n', msg)
+
+        finally:
+            SMTP.send = send