=== modified file 'Mailman/Handlers/SpamDetect.py'
--- Mailman/Handlers/SpamDetect.py	2012-02-05 21:37:29 +0000
+++ Mailman/Handlers/SpamDetect.py	2013-10-08 04:57:09 +0000
@@ -27,6 +27,7 @@
 
 import re
 
+from email.Errors import HeaderParseError
 from email.Header import decode_header
 
 from Mailman import mm_cfg
@@ -68,7 +69,10 @@
     headers = ''
     for h, v in msg.items():
         uvalue = u''
-        v = decode_header(re.sub('\n\s', ' ', v))
+        try:
+            v = decode_header(re.sub('\n\s', ' ', v))
+        except HeaderParseError:
+            v = [(v, 'us-ascii')]
         for frag, cs in v:
             if not cs:
                 cs = 'us-ascii'