PHP Fatal error: Class 'SoapClient' not found

If you're getting the error:

PHP Fatal error: Class 'SoapClient' not found

 .. it means the PHP SOAP client isn't installed on your server.

The Smart Send API is SOAP based, and the SDK and Smart Send-produced plugins use the default PHP SOAP client; however, it's not installed by default on some hosting environments.

Ask your hosting service to install it, or see http://php.net/manual/en/soap.installation.php