Well that was silly; additional work on #81.

Author: amcgregor

marrow/mailer/message.py

@@ -191,7 +191,7 @@ def _build_header_list(self, author, sender):
 		else:
 			headers.extend(self.headers)
 
-		if 'message-id' not in (i[0].lower() for header in headers):
+		if 'message-id' not in (header[0].lower() for header in headers):
 			headers.append('Message-Id', self.id)
 
 		return headers