Unixman posted a tutorial about installing Dovecot and Squirrelmail in CentOS & Scientific Linux

Although most versions of CentOS come standard with a POP3/IMAP server, dovecot can be much easier to configure and works almost straight after installation. Both the POP3 and IMAP protocols are primarily used for receiving emails on a mail server, almost all mail servers these days support both protocols; however there is a slight difference between the two. The main different between these protocols is that the POP3 protocol retrieves the whole email for the client, once

the emails are retrieved they removed from the server; where as a IMAP stores only the header information on the client side and changes to the email can be kept track of. There are some advantages and disadvantages of each protocol; however I will leave it up to your discretion which you would prefer to use.

First things first, you will need to start out by installing both Dovecot and Squirrelmail using yum.

  Installing Dovecot and Squirrelmail in CentOS & Scientific Linux