private function addressee_exists(string $addressee, array $lists = []): bool {
		foreach ($this->recipients as $recipient_list => $recipients) {
			if (in_array($recipient_list, $lists) or count($lists) === 0) {
				foreach ($recipients as $recipient) {
					if ($recipient['email'] == $addressee) {
						return true;
					}
				}
			}
		}

		return false;
	}