Two example implementations for wp_mail_failed

wp_mail_failed_handlers.php

// Echo out wp_mail() errors to the current page
// from https://developer.wordpress.org/reference/hooks/wp_mail_failed/
function echo_wp_mail_failed( $wp_error ) {
    echo "<pre>";
    print_r($wp_error);
    echo "</pre>";
}
add_action( 'wp_mail_failed', 'echo_wp_mail_failed', 10, 1 );

// Write wp_mail() errors to the debug log
function log_wp_mail_failed( $wp_error ) {
    error_log('wp_mail_failed: ' . print_r( $wp_error, true ));
}
add_action( 'wp_mail_failed', 'log_wp_mail_failed', 10, 1 );