Index: python/trunk/Lib/email/generator.py
===================================================================
--- python/trunk/Lib/email/generator.py	(revision 54016)
+++ python/trunk/Lib/email/generator.py	(working copy)
@@ -235,6 +235,16 @@
             print >> self._fp
             self._fp.write(msg.epilogue)
 
+    def _handle_multipart_signed(self, msg):
+        # The contents of signed parts has to stay unmodified in order
+        # to keep the signature intact, so we disable header wrapping.
+        old_maxheaderlen = self._maxheaderlen
+        try:
+            self._maxheaderlen = 0
+            self._handle_multipart(msg)
+        finally:
+            self._maxheaderlen = old_maxheaderlen
+
     def _handle_message_delivery_status(self, msg):
         # We can't just write the headers directly to self's file object
         # because this will leave an extra newline between the last header