diff -r a35b2d652449 Lib/email/encoders.py
--- a/Lib/email/encoders.py	Fri Dec 13 02:45:18 2013 +0100
+++ b/Lib/email/encoders.py	Fri Dec 13 10:07:30 2013 +0800
@@ -46,7 +46,6 @@
     msg['Content-Transfer-Encoding'] = 'quoted-printable'
 
 
-
 def encode_7or8bit(msg):
     """Set the Content-Transfer-Encoding header to 7bit or 8bit."""
     orig = msg.get_payload(decode=True)
@@ -54,25 +53,15 @@
         # There's no payload.  For backwards compatibility we use 7bit
         msg['Content-Transfer-Encoding'] = '7bit'
         return
-    # We play a trick to make this go fast.  If encoding/decode to ASCII
-    # succeeds, we know the data must be 7bit, otherwise treat it as 8bit.
+    # We play a trick to make this go fast.  If decoding to ASCII succeeds, we
+    # know the data must be 7bit, otherwise treat it as 8bit.
     try:
-        if isinstance(orig, str):
-            orig.encode('ascii')
-        else:
-            orig.decode('ascii')
+        orig.decode('ascii')
     except UnicodeError:
-        charset = msg.get_charset()
-        output_cset = charset and charset.output_charset
-        # iso-2022-* is non-ASCII but encodes to a 7-bit representation
-        if output_cset and output_cset.lower().startswith('iso-2022-'):
-            msg['Content-Transfer-Encoding'] = '7bit'
-        else:
-            msg['Content-Transfer-Encoding'] = '8bit'
+        msg['Content-Transfer-Encoding'] = '8bit'
     else:
         msg['Content-Transfer-Encoding'] = '7bit'
 
 
-
 def encode_noop(msg):
     """Do nothing."""