WordPress at a glance
action is not described

wp_mail_failed action-hook . WP 4.4.0

Fires after a phpmailerException is caught.

Usage

add_action( 'wp_mail_failed', 'action_function_name_4379' );
function action_function_name_4379( $error ){
	// action...
}
$error(WP_Error)
A WP_Error object with the phpmailerException message, and an array containing the mail recipient, subject, message, headers, and attachments.

Where the hook is called

wp_mail()
wp-includes/pluggable.php 495
do_action( 'wp_mail_failed', new WP_Error( 'wp_mail_failed', $e->getMessage(), $mail_error_data ) );
wp-includes/pluggable.php 363
do_action( 'wp_mail_failed', new WP_Error( 'wp_mail_failed', $e->getMessage(), $mail_error_data ) );

Where the hook is used (in WP core)

Does not used.