For future reference …
Had a few issues related to Horde sending mail. Although it appeared as though the mails were sent there was an issue with Horde using Sendmail.
One proposed solution was to edit php.ini and amend the disable_functions line in /etc/php.ini and remove the popen and escapeshellcmd functions so that PHP/Horde can use these for Sendmail.
disable_functions = symlink,shell_exec,exec,proc_close,proc_open,popen,system,dl,passthru,escapeshellarg,escapeshellcmd
$conf['mailer']['type'] = 'sendmail';
$conf['mailer']['type'] = 'smtp';