This is not a manual describing the installation (pre-requisites) of ISPConfig software on a Centos platform. An excellent manual can be found online. It's just that I ran into a problem when I tried to connect an e-mail client to the (IMAP) mailserver (controled by ISPConfig). All the appropriate ports / listeners were up and running, so it had to be a configuration issue.
yum install cyrus-sasl-plain-2.1.23-13.el6.x86_64
Software that handles cleartext passwords between mail processes. After the installation, the mail went flying across the Interwebs.