server admin fix postfix mailboxes, tidy up server, add better spam

I have a dedicated server with approx 15 websites. The server is running linux and plesk 10.2 The other day the qmail web server was attacked by a spam bot and 500k emails flooded the server and brought it down. I have managed to clear disk space, and change to postfix mail, but some of the mailboxes are not working correctly and mails to certain clients are sticking in deferred rather than being delivered. I need a server administrator to do the following - 1. fix all mailboxes so they work correctly 2. tidy up the server to maximise free space on var and usr areas 3. add some better spam protection for email server 4. restrict users from sending out bulk emails to many recipients and hundreds of emails to 5-6 people at a time it is important to not update plesk or php so none of the sites are broken and that server downtime is kept to a minimum.