Error : There was an error sending your message: Failed to open sendmail [/usr/lib/sendmail] for execution.

Make changes in : /home/kloxo/httpd/webmail/horde/config/conf.php

# vi /home/kloxo/httpd/webmail/horde/config/conf.php

change
$conf[‘mailer’][‘type’] = ‘sendmail’;
to
$conf[‘mailer’][‘type’] = ‘smtp’;

Restart your courier and qmail server.

service courier-imap stop
service qmail stop
service courier-imap start
service qmail start