Send Newsletter-Email when post changes

You can use a hook developed for such kind of purposes: post_updated

Use this hook whenever you need to compare values before and after the
post update.

Lets get some codding:

function mail_on_update($post_ID, $post_after, $post_before){
    if($post_after->post_content !=  $post_before->post_content){
        // wp_mail() , mail() here
    }
}

add_action( 'post_updated', 'mail_on_update', 10, 3 ); // 10 - priority, 3 - qty of parameters passed to action callback.

Put this code in your theme functions.php and modify with conditional logic for mailing.