Make sure email messages are always of data type bytes.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/sources/models.py

@@ -14,7 +14,7 @@ from django.core.files import File
 from django.core.files.base import ContentFile
 from django.db import models, transaction
 from django.utils.encoding import (
-    force_str, force_text, python_2_unicode_compatible
+    force_bytes, force_str, force_text, python_2_unicode_compatible
 )
 from django.utils.timezone import now
 from django.utils.translation import ugettext_lazy as _
@@ -566,7 +566,7 @@ class EmailBaseModel(IntervalBaseModel):
         from flanker import mime
 
         counter = 1
-        message = mime.from_string(force_str(message_text))
+        message = mime.from_string(force_bytes(message_text))
         metadata_dictionary = {}
 
         if not message_properties: