Blacklist (or move to junk folder) based on Subject: contents

==== Required information ====
- iRedMail version: 0.8.7
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Linux/BSD distribution name and version: FreeBSD 10.0
- Related log if you're reporting an issue:
====

How to blacklist a message based on a string in the email Subject: header? If this is not possible, can we use something like Sieve server-side rules to move messages into a Junk folder?

Thanks!