WordPress Hook to Notify Users When New Post Was Published

wp_post_publish_email_notify

/**
 * Send E-mails When a New Post is Published
 */
function email_notify( $new_status, $old_status, $post ) {

    // Avoids re-sending e-mails on update
    if( $new_status == 'publish' && $new_status != $old_status ) {

        $message = 'The post ' . $post->post_title . ' was published at ' . get_bloginfo('name');

        add_filter( 'wp_mail_content_type', 'set_html_content_type' );

        wp_mail( 'recipient@email.com', 'New Post Published At ' . get_bloginfo('name'), $message );

        // Reset content-type to avoid conflicts -- http://core.trac.wordpress.org/ticket/23578
        remove_filter( 'wp_mail_content_type', 'set_html_content_type' );

        return $post->ID;

    }

}
add_action( 'transition_post_status', 'email_notify', 10, 3 );

function set_html_content_type() {
    return 'text/html';
}