#match_email
  {
    margin-left:48px;
    overflow:auto; 
    display:none;
    width:200px;
    border:1px solid #aaa;
    background:#fff;
    max-height:100px;
    line-height:20px;
  }

  #match_email div
  {
    text-decoration:none;
    color:#000;
    width:200px;
  }

  #match_email div:hover
  {
    background:#aaa;  
  }

  input
  {
    height:20px;  
    width:200px;
  }

<div>
  邮箱：<input type="text" name="email" id="email" autocomplete="off" onkeyup="match_mail(this.value)"/>
  <div id="match_email"></div>

</div>

<script>
  //mailBoxs里存储用来匹配的串
  var mailBoxs = "@163.com @126.com @129.com"
  mailBoxs += " @qq.com @vip.qq.com @foxmail.com @live.cn @hotmail.com @sina.com @sina.cn @vip.sina.com";
  var matchmail = document.getElementById("match_email");
  var email = document.getElementById("email");
  function match_mail(keyword)
  {
    matchmail.innerHTML = "";
    matchmail.style.display = "none";
    if (!keyword)
      return;
    if (!keyword.match(/^[\w\.\-]+@\w*[\.]?\w*/))  
      return;
    keyword = keyword.match(/@\w*[\.]?\w*/);
    var matchs = mailBoxs.match(new RegExp(keyword+"[^ ]* ","gm"));
    if (matchs)
    {
      matchs = matchs.join("").replace(/ $/,"").split(" ");
      matchmail.style.display = "block";
      for (var i = 0; i < matchs.length; i++)
      {
        matchmail.innerHTML += '<div>'+matchs[i]+'</div>'; 
      }  
    }
  }
  //点击除了匹配框之外的任何地方，匹配框消失
  document.onclick = function(e)
  {
    var target = e.target;
    if (target.id != "matchmail")
      matchmail.style.display = "none";  
  }
  //将匹配框上鼠标所点的字符放入输入框
  matchmail.onclick = function(e)
  {
    var target = e.target;
    email.value = email.value.replace(/@.*/,target.innerHTML);
  }
</script>