Now, I might show SquirrelMail set up in some other post. For now, let us talk about setting up of OpenWebMail. Download the package from here and install it (just double clicking it in Ubuntu will be enough). You might need to install some perl packages. Fire the command if you need Perl:
sudo apt-get install perl
http://localhost/openwebmail/ will open the documentation and http://localhost/openwebmail/redirect.html will open the login page. Sweet and easy!