Fixed #19186 -- Fixed sending mail with unicode content on Python 3

Thanks alex_po for the report and Luke Plant for the analysis.

django/core/mail/backends/smtp.py

@@ -7,6 +7,7 @@ from django.conf import settings
 from django.core.mail.backends.base import BaseEmailBackend
 from django.core.mail.utils import DNS_NAME
 from django.core.mail.message import sanitize_address
+from django.utils.encoding import force_bytes
 
 
 class EmailBackend(BaseEmailBackend):
@@ -102,9 +103,11 @@ class EmailBackend(BaseEmailBackend):
         from_email = sanitize_address(email_message.from_email, email_message.encoding)
         recipients = [sanitize_address(addr, email_message.encoding)
                       for addr in email_message.recipients()]
+        message = email_message.message()
+        charset = message.get_charset().get_output_charset() if message.get_charset() else 'utf-8'
         try:
             self.connection.sendmail(from_email, recipients,
-                    email_message.message().as_string())
+                    force_bytes(message.as_string(), charset))
         except:
             if not self.fail_silently:
                 raise