Help fix compromised Linux server and fix email issue

Hi all, Preamble: I have two Linode servers with a VPS provider called Linode ([login to view URL]). Server A runs CentOS Linux 6.5 and Server B runs Ubuntu Linux 12.04.3. Both servers run web apps with Apache web server. Server A has a mail system that was set up with by tying up Dovecot, MySQL, Posffix and PostfixAdmin. So, emails for the various domains hosted on the server are managed through PostFixAdmin. Server B’s mail server too was configured using Dovecot, MySQL, Postfix and PostfixAdmin, but it is not working properly, so I configured server B to use Server A’s email servers to process emails for the various domains, by indicating server A’s email server in Linode’s DNS configurations for domains hosted on server B. Only one web application, OpenERP runs on server B. (Although, I have set up virtual hosting on it on Apache. The problem: Some weeks ago, I granted someone access to server B to try to install a new version of OpenERP, which is now called Odoo on server B. He ran a script to install it and it didn’t work, because the minimum requirement was Ubuntu 14.0. Shortly afterwards, Linode ([login to view URL]) the VPS hosting company) alerted me to the fact that my server was sending out spam mails. As a result, mails from all the domains hosted on the CentOS server have stopped getting delivered to their targets. I believe that this is because the IP address has been blacklisted. I shut down server B to stop the spam mails from being sent from my server but they were still going from server B. I also found out that the root password to server B was changed. I have now changed it back to the original. Required solution: #1. Identify the cause of the spam mails being generated and remove it. I suspect that something has been put on the server that is causing this. #2. Help tie up Dovecot, MySQL, Postfix, PostfixAdmin properly on server B. #3. Help find or propose a solution to the non-delivery of emails from server B (probably caused by the suspected blacklisting of the server IP address) #4. Help migrate a website (a wordpress site) from Server A to server B. The site should be setup on server B using virtual web hosting. The total budget for this project is $35. Thanks.