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

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

django/core/mail/message.py

@@ -25,11 +25,15 @@ from django.utils.encoding import force_text
 # some spam filters.
 utf8_charset = Charset.Charset('utf-8')
 utf8_charset.body_encoding = None  # Python defaults to BASE64
+utf8_charset_qp = Charset.Charset('utf-8')
+utf8_charset_qp.body_encoding = Charset.QP
 
 # Default MIME type to use on attachments (if it is not explicitly given
 # and cannot be guessed).
 DEFAULT_ATTACHMENT_MIME_TYPE = 'application/octet-stream'
 
+RFC5322_EMAIL_LINE_LENGTH_LIMIT = 998
+
 
 class BadHeaderError(ValueError):
     pass
@@ -169,7 +173,10 @@ class SafeMIMEText(MIMEMixin, MIMEText):
             # We do it manually and trigger re-encoding of the payload.
             MIMEText.__init__(self, _text, _subtype, None)
             del self['Content-Transfer-Encoding']
-            self.set_payload(_text, utf8_charset)
+            has_long_lines = any(len(l) > RFC5322_EMAIL_LINE_LENGTH_LIMIT for l in _text.splitlines())
+            # Quoted-Printable encoding has the side effect of shortening long
+            # lines, if any (#22561).
+            self.set_payload(_text, utf8_charset_qp if has_long_lines else utf8_charset)
             self.replace_header('Content-Type', 'text/%s; charset="%s"' % (_subtype, _charset))
         elif _charset is None:
             # the default value of '_charset' is 'us-ascii' on Python 2

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'])