项目中有一个需求是监控系统性能并发送邮件，考虑到配置代理邮箱发送邮件太重量级，并且繁琐复杂，频繁发送邮件还有IP被禁之虞。所以选择sendmail来发送邮件，简单粗暴。
sendmail的使用方法
linux下安装sendmail和sendmail.service，启动sendmail.service。貌似使用mail命令发送邮件更多，使用sendmail命令发送html格式邮件的方法如下：
echo '<html><body><h4>当前系统中产生一条告警信息，请您知晓</h4></body></html>'|formail -I "From: service" -I "To:$mailList" -I "MIME-Version:1.0" -I "Content-type:text/html;charset=utf-8" -I "Subject:monitor" |sendmail -toi
$mailList是收件人地址，如果由多个收件人，用”,”分隔。有时使用这个命令会被当作垃圾邮件，这时使用sendmail -toi $mailList会解决。其他诸如乱码问题可通过设置编码来解决，参考sendmail+formail乱码这篇文章。
formail命令用来组装邮件头。可以看一下formail之后的邮件格式：
From: service
To: test@163.com
MIME-Version:1.0
Content-type:text/html;charset=utf-8
Subject:monitor
<html><body><h4>当前系统中产生一条告警信息，请您知晓</h4></body></html>
所以我们可以把邮件按照上述格式写到文件里，然后通过cat mail.txt|sendmail -toi发送。
如果你的hostname为默认的localhost的话，你可能会得到一个错误：dsn=5.0.0, stat=Service unavailable，这是由于默认使用root@localhost.localdomain发件，绝大多数邮箱都会拒收。
解决办法：修改hostname，修改/etc/hosts文件，添加你的hostname;修改/etc/sysconfig/network文件，改成HOSTNAME=yourhostname，重启sendmail.service服务即可。
通过java调用sendmail
通过java调用shell命令的方法是Runtime.getRuntime().exec(command)，下面直接上代码：
package com.vosamo;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.text.MessageFormat;
import java.text.SimpleDateFormat;
import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class OSMailSender {
private List<String> to;
private String from;
private String subject;
private String content;
public OSMailSender(List<String> to, String from, String subject, String content) {
this.to = to;
this.from = from;
this.subject = subject;
this.content = content;
}
public String listToString(List<String> toList) {
if (toList==null) {
return null;
}
StringBuilder result=new StringBuilder();
boolean flag=false;
for (String to : toList) {
if (flag) {
result.append(",");
}else {
flag=true;
}
result.append(to);
}
return result.toString();
}
public void callShell(String shellString) {
String[] command = {"/bin/sh", "-c", shellString};
System.out.println(command.toString());
try {
Process process = Runtime.getRuntime().exec(command);
BufferedReader input = new BufferedReader(new InputStreamReader(process.getInputStream()));
String line = "";
while ((line = input.readLine()) != null) {
line += line; //获取命令执行结果
}
input.close();
int exitValue = process.waitFor(); //获取命令执行返回状态码
if (0 != exitValue) {
System.out.println("call shell failed. error code is :" + exitValue);
System.out.println("Reason: " + line);
}
} catch (Throwable e) {
System.out.println("call shell failed. " + e);
}
}
public void send() {
String mailMessage =
"SUBJECT: {0} \n" //邮件标题
+ "TO: {1} \n" //收件人
+ "FROM: {2} \n" //发件人
+ "MIME-VERSION: 1.0 \n"
+ "Content-type: text/html \n"
+ "{3}"; //邮件正文
String receiver = listToString(to);
mailMessage = MessageFormat.format(mailMessage, this.subject, receiver, this.from, this.content);
String mailCommand = "echo '" + mailMessage + "'| sendmail -toi";
callShell(mailCommand);
}
public static void main(String[] args) {
List<String> toList = new ArrayList<String>();
toList.add("test1@163.com");
toList.add("test2007@163.com");
String from = "buptlsl@163.com";
String subject = "Unicorn监控平台告警 级别：严重警告";
String content = "<html><body><h4>当前系统中产生一条告警信息，请您知晓</h4></body></html>";
OSMailSender sender = new OSMailSender(toList, from, subject, content);
sender.send();
}
}sendmail的反向解析
测试时发现，调用sendmail发邮件，发着发着突然收不到邮件了。于是查看sendmail的状态：
systemctl status sendmail查看状态和sendmail -bp查看sendmail队列，结果发现：发邮件的请求都在队列里面排队积压，就是发不出去，如下：
[root@mycentos ~]# sendmail -bp
:call <SNR>110_SparkupNext()
/var/spool/mqueue (22 requests)
-----Q-ID----- --Size-- -----Q-Time----- ------------Sender/Recipient-----------
u9Q36LgF014458* 469 Wed Oct 26 11:08 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q37L92014515 469 Wed Oct 26 11:09 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q38LYx014576 469 Wed Oct 26 11:10 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q39L8e014657 469 Wed Oct 26 11:11 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q3ALl5014719 469 Wed Oct 26 11:12 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q3BLY7014775 469 Wed Oct 26 11:13 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q3CLwF014832 469 Wed Oct 26 11:14 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q3DLlR014890 469 Wed Oct 26 11:15 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q3ELVj014995 469 Wed Oct 26 11:16 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q3FLJV015053 469 Wed Oct 26 11:17 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q3GLal015110 469 Wed Oct 26 11:18 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
<vosamo007@163.com>
<buptlsl@163.com>
u9Q3HLwh015220 469 Wed Oct 26 11:19 <root@mycentos.com>
8BITMIME (host map: lookup (163.com): deferred)
其中的host map: lookup (163.com): deferred，这表示发送邮件出现了延迟，网上说导致这个问题的原因是sendmail默认会进行反向解析，而我自己的机器上没有合法的域名和MX解析记录。解决的办法很简单，打开文件/etc/mail/sendmail.cf，找到#0 ResolverOptions=+AAONLY这一行，把注释去掉就可以了。虽然问题是解决了，但是还不是很明白其中的原因，为什么反向地址解析和发邮件延迟会有关系呢？后续研究一下这个问题再写一篇文章吧。
另外，邮件队列中积压的请求删除的话，只需要把/var/spool/mqueue/下面的文件全部删除就可以了。