从php发送电子邮件不起作用

我制作了一个用于android设备的工作注册脚本。唯一剩下的就是向用户发送一封欢迎他的电子邮件，即"欢迎使用该应用程序…"等等。然而，我对此感到困惑，因为没有发送电子邮件。

这是我的剧本。

<?php
   require "init.php";
   header('Content-type: application/json');
  $email = $_POST['email'];
  $user_name = $_POST['user_name'];
  $user_pass = $_POST['user_pass'];
  $msg = "Congratulations. You are now registered to the most amazing app    
  ever!";
  $passwordEncrypted = sha1($user_pass);         
    if($email && $user_name && $user_pass){
        $sql_query = "select * from user_info WHERE email ='".mysqli_real_escape_string($con, $email)."' and user_name ='".mysqli_real_escape_string($con, $user_name)."'";
        $result = mysqli_query($con, $sql_query);   
        $results = mysqli_num_rows($result);
        if ($results){
            $don = array('result' =>"fail","message"=>"Email or username exists.");
        }else{
            $sql_query = "insert into user_info values('$email','$user_name','$passwordEncrypted');";
            if(mysqli_query($con,$sql_query)){
                $don = array('result' =>"success","message"=>"Successfully registered!Well done");
                mail("$email","Well done",$msg);
            }
        }
    } if(!$email || !filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)){
        $don = array('result' =>"fail","message"=>"Please enter a valid email");
    }else if(!$user_name){
        $don = array('result' =>"fail","message"=>"Please enter your username");
    }else if(!$user_pass || (strlen($user_pass)<8 || (!preg_match('`[A-Za-z0-9]`', $user_pass)))){
        $don = array('result' =>"fail","message"=>"Password must contain 8 letters or more with at least one capital letter ");
    }

  echo json_encode($don);
?>

有什么想法吗？

谢谢。