Configuring the sendmail transport with the Mail resource causes it to be constructed incorrectly, which ends up throwing an exception when used:

resources.mail.transport.type = "Sendmail"

Exception:

Zend_Mail_Transport_Sendmail expects a string as it's constructor argument, but the Zend_Application_Resource_Mail resource passes in an array.

// line 128
            case 'Zend_Mail_Transport_Sendmail':
            default:
                $transport = new $transportName($options);

One solution would be to implement the sendmail case and use a single option to pass in to the constructor:

// line 128
            case 'Zend_Mail_Transport_Sendmail':
                $transport = new $transportName($options['options']);
                break;
            default:
                $transport = new $transportName($options);