Wraps long lines in bulk email messages.

RFC 2822 states that line lengths in emails must be less than 998.
Some MTA's add newlines to messages if any line exceeds a certain
limit (the exact limit varies). Sendmail goes so far as to add
'!\n' after the 990th character in a line. To ensure that bulk
mail messages look consistent long lines are wrapped to a
conservative length.

LMS-1466

common/lib/mail_utils.py

@@ -0,0 +1,22 @@
+"""
+Utilities related to mailing.
+"""
+
+import textwrap
+
+MAX_LINE_LENGTH = 900
+
+
+def wrap_message(message, width=MAX_LINE_LENGTH):
+    """
+    RFC 2822 states that line lengths in emails must be less than 998. Some MTA's add newlines to messages if any line
+    exceeds a certain limit (the exact limit varies). Sendmail goes so far as to add '!\n' after the 990th character in
+    a line. To ensure that messages look consistent this helper function wraps long lines to a conservative length.
+    """
+    lines = message.split('\n')
+    wrapped_lines = [textwrap.fill(
+        line, width, expand_tabs=False, replace_whitespace=False, drop_whitespace=False, break_on_hyphens=False
+    ) for line in lines]
+    wrapped_message = '\n'.join(wrapped_lines)
+
+    return wrapped_message

lms/djangoapps/bulk_email/models.py

@@ -12,11 +12,12 @@ file and check it in at the same time as your model changes. To do that,
 
 """
 import logging
-from django.db import models, transaction
-from django.contrib.auth.models import User
-from html_to_text import html_to_text
-
 from django.conf import settings
+from django.contrib.auth.models import User
+from django.db import models, transaction
+
+from html_to_text import html_to_text
+from mail_utils import wrap_message
 
 log = logging.getLogger(__name__)
 
@@ -192,8 +193,8 @@ class CourseEmailTemplate(models.Model):
         message_body_tag = COURSE_EMAIL_MESSAGE_BODY_TAG.format()
         result = result.replace(message_body_tag, message_body, 1)
 
-        # finally, return the result, without converting to an encoded byte array.
-        return result
+        # finally, return the result, after wrapping long lines and without converting to an encoded byte array.
+        return wrap_message(result)
 
     def render_plaintext(self, plaintext, context):
         """