Fixed #27696 -- Measured email long lines on encoded content

Thanks Pavel Pokrovskiy for the report and Tim Graham for the review.

django/core/mail/message.py

@@ -219,7 +219,10 @@ class SafeMIMEText(MIMEMixin, MIMEText):
 
     def set_payload(self, payload, charset=None):
         if charset == 'utf-8':
-            has_long_lines = any(len(l) > RFC5322_EMAIL_LINE_LENGTH_LIMIT for l in payload.splitlines())
+            has_long_lines = any(
+                len(l.encode('utf-8')) > RFC5322_EMAIL_LINE_LENGTH_LIMIT
+                for l in payload.splitlines()
+            )
             # Quoted-Printable encoding has the side effect of shortening long
             # lines, if any (#22561).
             charset = utf8_charset_qp if has_long_lines else utf8_charset

tests/mail/tests.py

@@ -779,7 +779,8 @@ class BaseEmailBackendTests(HeadersCheckMixin, object):
         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'])
+        # Unencoded body length is < 998 (840) but > 998 when utf-8 encoded.
+        email = EmailMessage('Subject', 'В южных морях ' * 60, 'from@example.com', ['to@example.com'])
         email.send()
         message = self.get_the_message()
         self.assertMessageHasHeaders(message, {