Send automatic emails to multiple user roles when a custom field is updated in a custom post type

This code will allow you to call a custom function when your meta key is updated.

add_action('updated_postmeta','my_meta_update', 10, 4);

function my_meta_update( $meta_id, $object_id, $meta_key, $meta_value ) {

    if($meta_key === 'my_meta_key') {
        // trigger email(s)
        // wp_mail( $to, $subject, $message, $headers, $attachments );
    }
}

Like Shaun mentioned, you should always use an external email solution else your ip / domain will get blacklisted. You may even have email sending restrictions from your hosting provider.