private function fromArrayToString(array $fromArray): string
    {
        // $fromArray = [ 'john.doe@example.com' => 'John Doe', 'jane.doe@example.com' => 'Jane Doe' ]; // sample
        if (count($fromArray) > 0) {
            reset($fromArray);
            $fromString = key($fromArray);

            return $fromString;
        }

        throw new \InvalidArgumentException('Set at least one \'from\' address when using '.self::class);
    }