Linux Server postfix configuration & Maybe Fail2ban installation

Problem: Our server has been targeted to send a large number of spam emails which times out all sites on our server. we have to re-boot our server which can cause some site issues each time. each time our server slows down i get a text message to alert me that the server has timed out. everytime i check the mail queue 1000's of spam emails are in there so we delete via ssh Security so far: We have some hardening of server which only allows our ip address to access plesk, we have also installed watchdog Requirements: We have been informed we need to have the postfix configured to only allow mailboxes on server can send mail, we have also been informed fail2ban maybe good also WE ARE NO EXPERT ON THIS ANYONE WHO KNOWS HOW TO TACKLE THIS ISSUE PLEASE QUOTE THE PRICE FOR WHAT YOU THINK NEEDS TO BE DONE THEN SEND US A MESSAGE WITH WHAT YOU THINK IS REQUIRED We have a feeling one of our websites maybe be hacked but we have downloaded all files and scanned with avast Example of spam: Received: by (removed our server ip is here)(Postfix, from userid xxx) id xxxxxxx; Fri, 26 Oct 2012 19:34:37 +0100 (BST) To: "xxxxxxxxxx" <xxxxxxxx> Subject: Your friend ...... OPEN TO WIN THE BIG LOTTO! ......... has recommended this great product from (our company name) X-PHP-Originating-Script: 10000:[login to view URL] From: "...... OPEN TO WIN THE BIG LOTTO! ........." <xxxxxxxxxxxxx> MIME-Version: 1.0 X-Mailer: osCommerce bulk mailer Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Message-Id: <20121026183437.981299043@xxxxxxxxxxxxxxxxxx> Date: Fri, 26 Oct 2012 19:34:37 +0100 (BST) https://xxxxxxxxx/server/[login to view URL]