IMAP offers the flexibility that most people want with email. Access mail from different computers, locations and also access from PDAs, TV, etc. IMAP by default stores mail on a central server that provides access to a mail account from anywhere in the world.
SquirrelMail is a standards-based program that offers you an easy way to set up an IMAP web based email server. It is written in PHP4 and provides support for both IMAP and SMTP. One advantage of SquirrelMail is that the javaScript is an option not a requirement for users making it more browser compatible. The setup on a  CentOS or RHEL  server is an effective way to provide email access to roaming users from an office or from home.
Requirements
In order to use Squirrelmail you must have installed:
Apache
PHP4.1.2 or better
IMAP
Browser – cookies must be enabled
Install
SquirrelMail has become an option for install on most Linux distributions.

Centos
yum install squirrelmail

Ubuntu
apt-get install squirrelmail
SquirrelMail is installed in /usr/share/squirrelmail typically.
Some directories for queuing & attachments are set up in /var/lib.
The config files are found in /etc/squirrelmail.
A config file for httpd is added to /etc/http/conf.d which creates the alias, webmail for SquirrelMail. The webmail alias allows you to access SquirrelMail by using the link, http://example.org/webmail
This will then bring up the login for the server. A cron job is activated to purge the attachments directory.
/etc/cron.daily/squirrelmail.cron