Yes, it's possible. To do it, you'll need to edit the template for imp's conf.php file. Edit /etc/e-smith/templates/home/httpd/html/horde/imp/config/conf.php/40Conf and find the line that says
$conf['user']['redirect_on_logout'] = false;
change "false" to whatever URL you want to use (probably should be in quotes). Then expand the template, and you're set.