Use hidden config, and actual CP setting in Email Configuration to send emails as whatever format the config value is set as. Ideal for sending notification emails as HTML.

librariesEE_Email.php

function EE_initialize()
{   
    $config = array(
                    'protocol'      => ( ! in_array( $this->EE->config->item('mail_protocol'), $this->_protocols)) ? 'mail' : $this->EE->config->item('mail_protocol'),
                    'charset'       => ($this->EE->config->item('email_charset') == '') ? 'utf-8' : $this->EE->config->item('email_charset'),
                    'smtp_host'     => $this->EE->config->item('smtp_server'),
                    'smtp_user'     => $this->EE->config->item('smtp_username'),
                    'smtp_pass'     => $this->EE->config->item('smtp_password'),
                    'mailtype'      => $this->EE->config->item('mail_format')
                    );
    
    /* -------------------------------------------
    /*  Hidden Configuration Variables
    /*  - email_newline => Default newline.
    /*  - email_crlf => CRLF used in quoted-printable encoding
    /*  - email_smtp_port => SMTP Port
    /* -------------------------------------------*/
    
    if ($this->EE->config->item('email_newline') !== FALSE)
    {
        $config['newline'] = $this->EE->config->item('email_newline');
    }
    
    if ($this->EE->config->item('email_crlf') !== FALSE)
    {
        $config['crlf'] = $this->EE->config->item('email_crlf');
    }

    if ($this->EE->config->item('email_smtp_port') !== FALSE)
    {
        $config['smtp_port'] = $this->EE->config->item('smtp_port');
    }

    if ($this->EE->config->item('email_message_format') !== FALSE)
    {
        $config['mailtype'] = $this->EE->config->item('email_message_format');
    }
            
    $this->useragent = APP_NAME.' '.APP_VER;        

    $this->initialize($config);
}