Dedicated Server Email Setup

We have recently purchased a dedicated server running Fedora Core 2 and PHP4. The server has Plesk 7.5.4 Installed The email system that is being assigned to webmail accounts is Horde. We recently had the MySql 3 version updated to MySql 4 . Horde seems to be working and we can create email accounts, we just can not send or receive from them. I am attaching a document with all of the know components installed. We are busy setting up 3 other servers and only have two people workin g on setting everything up. This should be a very easy fix for an experienced systems admin. To be considered you must have at least 4 Excellent Ratings.