根据邮件地址获得邮件服务器

	/**
	 * 根据邮件地址，获得邮件服务器
	 * @param mail
	 * @return
	 * lupingui
	 * 2009-4-17
	 */
	public static String getSmtpHost(String mail){
		//获得邮件Domain
		int atMarkPosition = mail.indexOf("@");
		if (atMarkPosition < 1){
			return null;
		}
		String mailDomain = mail.substring(atMarkPosition + 1);
		
		boolean isMxSuccessful = false;
		//查询邮件交换记录
		Lookup lookup = null;
		try {
			lookup = new Lookup(mailDomain, Type.MX);
			lookup.run();
			if(lookup.getResult() == Lookup.SUCCESSFUL){
				isMxSuccessful = true;
			}else{				
				lookup = new Lookup(mailDomain);
				lookup.run();
				if(lookup.getResult() != Lookup.SUCCESSFUL){
					return null;
				}
			}
		} catch (TextParseException ex) {
			ex.printStackTrace();
			return null;
		}
		
		//得到服务器列表
		Record[] answers = lookup.getAnswers();
		String[] hostArray = new String[answers.length];
		for (int i = 0; i < answers.length; i++) {
			Record record = answers[i];
			Name name = record.getAdditionalName();
			if(!isMxSuccessful){
				name = record.getName();
			}
			int labelCount = name.labels();
			StringBuffer sbf = new StringBuffer();
			for (int j = 0; j < labelCount; j++) {
				String labelString = name.getLabelString(j);
				if(!labelString.equals("")){
					sbf.append(labelString).append(".");
				}
			}
			if(sbf.length() > 0 && sbf.charAt(sbf.length()-1) == '.'){
				sbf.deleteCharAt(sbf.length()-1);
			}
			hostArray[i] = sbf.toString();
		}
		
		//返回，如果有多个则随机返回一个。
		if(hostArray.length == 1){
			return hostArray[0];
		}else{
			Random random = new Random();
			int index = random.nextInt(hostArray.length);
			return hostArray[index];
		}
	}