private String getSubdomainOfNatural(String _recipient) {
		String recipient = new String();
		//2つの#が含まれていれば,その間の文字列を取り除く
		int lastSharpIndex = _recipient.lastIndexOf("#");
		if (lastSharpIndex > 0)
			recipient = _recipient.substring(lastSharpIndex + 1);
		else if (lastSharpIndex < 0)
			recipient = _recipient;
		else
			return null;
		
		//ルールの数
		int ruleNum;
		
		int atIndex = recipient.indexOf("@");
		//@より前
		String account = recipient.substring(0, atIndex);
		//@より後
		String fullDomain = recipient.substring(atIndex + 1);
			
		//accountに"."があれば分割
		if (account.contains(".")) {
			String[] accounts = account.split("\\.");
			ruleNum = accounts.length;
		} else if (!account.isEmpty())
			ruleNum = 1;
		else
			ruleNum = 0;
			
		//ドメインの中の"."の数はルール数より大きくなければいけない.そうでなければnullを返す
		int dotNum = (fullDomain.split("\\.").length) - 1;	//ドメインの中の"."の数
		if (dotNum > ruleNum) {
			//ex)ルール数が2なら3つめの"."以降がドメイン名である.
			//rule1.rule2.subdomain.domain
			int startIndex = searchChr(fullDomain, ".", ruleNum) + 1;
			int endIndex = searchChr(fullDomain, ".", ruleNum + 1);

			if ((ruleNum == 0 || startIndex > 0) && endIndex > startIndex) {
				String subdomain = fullDomain.substring(startIndex, endIndex);
				return subdomain;
			} else
				return null;
		} else
			return null; 
	}