Try setting your sendmail_path (in your 'php.ini file) to just '/usr/sbin/sendmail'(without the -t -i switches) to see if this makes a difference.
Maybe your web host could shed a little more light on the "Could not execute: /usr/sbin/sendmail -t -i" error message.
Otherwise, try using SMTP instead.
Open the 'showkase/admin/plugins/contact/master/contactcore/contact.php' file in a plain text editor and change line 21 from:
$mailer->isMail();... to:
$mailer->isSMTP(); // Use SMTP
$mailer->SMTPDebug = 2; // Enable SMTP class debug output mode - can be 0, 1, 2, 3 or 4
$mailer->SMTPAuth = true; // Use SMTP authentication - can be true or false
$mailer->SMTPSecure = ''; // Secure connection prefix - can be '', 'ssl' or 'tls'
$mailer->Host = 'smtp.example.com'; // Set SMTP host
$mailer->Port = 25; // Set SMTP port
$mailer->Username = 'username'; // Set SMTP username
$mailer->Password = 'password'; // Set SMTP passwordYou will need to fill in your own unique SMTP details (as noted in the comments within the code).
You might also need to ensure that the 'From' address is a valid email address on your domain.
Try changing line 91 from:
$mailer->setFrom('mailer'.strstr($siteEmail, '@'), $userName);... to:
$mailer->setFrom('your@email.address', $userName);I hope this helps.