Send email only upon draft

$post_id is an integer (just the post id) and not a post object (the whole post with id,status,title …)

so globalize the $post object and check the status from there eg:

function er_send_email_on_post_draft_save( $post_id ) {
    global $post;
    //verify post is not a revision
    if ( $post->post_status == 'draft' ) {

        $post_title = get_the_title( $post_id );
        $post_url = get_permalink( $post_id );
        $subject="A post has been updated";

        $message = "A post has been updated on your website:\n\n";
        $message .= "" .$post_title. "\n\n";

        //send email to admin
        wp_mail( '[email protected]', $subject, $message );

    }
}

and if you want you can use a hook fired only when post is saved as draft

add_action('draft_post', 'send_my_mail_on_draft' );
function send_my_mail_on_draft( $post_id,$post){
   $post_title = get_the_title( $post_id );
   $post_url = get_permalink( $post_id );
   $subject="A post has been updated";

   $message = "A post has been updated on your website:\n\n";
   $message .= "" .$post_title. "\n\n";

   //send email to admin
   wp_mail( '[email protected]', $subject, $message );
}