diff -r faca3f5c1848 mail/rfc822_mailbox.py
--- a/mail/rfc822_mailbox.py	Tue Nov 10 10:41:41 2009 -0800
+++ b/mail/rfc822_mailbox.py	Fri Mar 11 13:41:18 2011 -0800
@@ -18,6 +18,8 @@
 decode_header # quiet checker.
 
 def encode_header(s):
+    if s == '':
+        s += '\n'
     return Header(stringify(s)).encode()
 
 class RFC822Mailbox: