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