Fixed #22561 -- Prevented too long lines in email messages

Thanks NotSqrt for the excellent report and Tim Graham for the review.

tests/mail/tests.py

@@ -639,6 +639,22 @@ class BaseEmailBackendTests(HeadersCheckMixin, object):
         self.assertEqual(message["subject"], '=?utf-8?q?Ch=C3=A8re_maman?=')
         self.assertEqual(force_text(message.get_payload(decode=True)), 'Je t\'aime très fort')
 
+    def test_send_long_lines(self):
+        """
+        Email line length is limited to 998 chars by the RFC:
+        https://tools.ietf.org/html/rfc5322#section-2.1.1
+        Message body containing longer lines are converted to Quoted-Printable
+        to avoid having to insert newlines, which could be hairy to do properly.
+        """
+        email = EmailMessage('Subject', "Comment ça va? " * 100, 'from@example.com', ['to@example.com'])
+        email.send()
+        message = self.get_the_message()
+        self.assertMessageHasHeaders(message, {
+            ('MIME-Version', '1.0'),
+            ('Content-Type', 'text/plain; charset="utf-8"'),
+            ('Content-Transfer-Encoding', 'quoted-printable'),
+        })
+
     def test_send_many(self):
         email1 = EmailMessage('Subject', 'Content1', 'from@example.com', ['to@example.com'])
         email2 = EmailMessage('Subject', 'Content2', 'from@example.com', ['to@example.com'])