Rainloop is an attractive webmail with a nice clean modern feel to it. After following several different installation procedures, I finally got a working copy.

Assumptions: Apache has already been installed. Sendmail or Postfix is already installed correctly.

Install Rainloop

mkdir /var/www/html/rainloop
cd /var/www/html/rainloop
curl -s http://repository.rainloop.net/installer.php | php
find . -type d -exec chmod 755 {} \;
find . -type f -exec chmod 644 {} \;
chown -R apache:apache .

Create a configuration file for Apache.

vim /etc/httpd/conf.d/20-rainloop.conf

Alias /webmail /var/www/html/rainloop

<Directory /var/www/html/rainloop>
Options -Indexes
AllowOverride All
</Directory>

<Directory /var/www/html/rainloop/data>
Order Deny,Allow
Deny from All
</Directory>

Restart or Reload Apache

service httpd reload
service httpd restart

Logon to the admin control panel with default credentials.
http://yourdmain.com/webmail/?admin
user: admin
pass: 12345

Among the settings on the add Domains page, use 127.0.0.1 for both with standard ports. Also ensure that there is an [x] for use short login forms and use authentication.

Source(s)
http://www.rainloop.net/docs/installation/
http://trac.roundcube.net/attachment/wiki/Howto_Install/centos_rhel_install.sh
http:.//xmodulo.com/rainloop-webmail-ubuntu-server.html
http:.//www.tecmint.com/rainloop-webmail-a-modern-fast-web-based-email-client-for-linux/