plugin maildir: fixes missing import

src/plugins/plugin_misc_maildir.py

@@ -29,6 +29,7 @@ from twisted.cred import portal, checkers
 from twisted.mail import imap4, maildir
 from email.parser import Parser
 import email.message
+import email.utils
 from email.charset import Charset
 import os
 from cStringIO import StringIO