diff --git a/Lib/MimeWriter.py b/Lib/MimeWriter.py
index 9f04656..6aab1cd 100644
--- a/Lib/MimeWriter.py
+++ b/Lib/MimeWriter.py
@@ -10,6 +10,7 @@
 import string
 import mimetools
 
+__all__ = ["MimeWriter"]
 
 class MimeWriter: