WordPress at a glance
action is not described

wp_mail_failed action-hook . WP 1.0

This filter is documented in wp-includes/pluggable.php

Usage

add_action( 'wp_mail_failed', 'action_function_name_9455' );
function action_function_name_9455( $WP_Error ){
	// action...
}
$WP_Error
-

Where the hook is called

wp_mail()
wp_mail_failed
wp-includes/pluggable.php 370
do_action( 'wp_mail_failed', new WP_Error( 'wp_mail_failed', $e->getMessage(), $mail_error_data ) );
wp-includes/pluggable.php 512
do_action( 'wp_mail_failed', new WP_Error( 'wp_mail_failed', $e->getMessage(), $mail_error_data ) );

Where in WP core the hook is used WordPress

Usage not found.