MAIL ISSUES - CENT OS 6 AND WHM/CPANEL - URGENT

We just finished a migration from an older server to a new one, and my tech has finally gone to sleep, but I just discovered an issue with mail on one of the accounts. I'm able to work with someone and provide testing and access support, but ultimately I need a WIZARD technically, and that's not me. The server is running now, it's a dedicated server running CentOS 6, and WHM, latest release. The mail is Dovecot and one of the accounts in particular is given problems. On that account we are using SquirrelMail. I need a wizard. Please only reply if you can help now, and are an absolute expert at troubleshooting these servers. Thank you!!