src/StatusCode.php
public static function getTranslationKeys(): array
    {
        return [
            self::UNSENT => 'unsent',
            self::SENT => 'sent',
            self::ACCEPTED => 'accepted',
            self::REJECTED => 'rejected',
            self::DELIVERED => 'delivered',
            self::FAILED => 'failed',
        ];
    }
public static function getTranslationKeys(): array
    {
        return [
            self::UNSENT => 'unsent',
            self::SENT => 'sent',
            self::ACCEPTED => 'accepted',
            self::REJECTED => 'rejected',
            self::DELIVERED => 'delivered',
            self::FAILED => 'failed',
        ];
    }
Failed and will not be delivered (CM state)