Need help with PHP mail script

Question

I  have created a  log in system  . In that when the  user completes the  registration process an auto reply(auto-reply@domain)  will generate and  sent this to users email id regarding about the user  name and password  (Lo gin Details). After formal approval from the admin  the user will  get a user activation mail with log in link.
But , my problem is  these are work only for mail accounts from my  domain  only(test@domain). its not send any of above mentioned details  to other  mail services like gmail or yahoo etc.
i discussed this   with some others, they said its the problem with your mail function   configuration. but i didn't get any needful information as am a  beginner  in PHP scripting.
i have contacted this with my  hosting service they said its the  problem with  php mail () function  and use php mailer() instead mail().
please give me a solution for the same..
Here am ataching my code..
<?php
include 'dbc.php';
$err = array();
if($_POST['doRegister'] == 'Register')
foreach($_POST as $key => $value) {
    $data[$key] = filter($value);
if(empty($data['full_name']) || strlen($data['full_name']) < 4)
$err[] = "ERROR - Invalid name. Please enter atleast 3 or more characters for your name";
//header("Location: register.php?msg=$err");
//exit();
// Validate User Name
if (!isUserID($data['user_name'])) {
$err[] = "ERROR - Invalid user name. It can contain alphabet, number and underscore.";
//header("Location: register.php?msg=$err");
//exit();
// Validate Email
if(!isEmail($data['usr_email'])) {
$err[] = "ERROR - Invalid email address.";
//header("Location: register.php?msg=$err");
//exit();
// Check User Passwords
if (!checkPwd($data['pwd'],$data['pwd2'])) {
$err[] = "ERROR - Invalid Password or mismatch. Enter 5 chars or more";
//header("Location: register.php?msg=$err");
//exit();
$user_ip = $_SERVER['REMOTE_ADDR'];
// stores sha1 of password
$sha1pass = PwdHash($data['pwd']);
// Automatically collects the hostname or domain  like example.com)
$host  = $_SERVER['HTTP_HOST'];
$host_upper = strtoupper($host);
$path   = rtrim(dirname($_SERVER['PHP_SELF']), '/\\');
// Generates activation code simple 4 digit number
$activ_code = rand(1000,9999);
$usr_email = $data['usr_email'];
$user_name = $data['user_name'];
$rs_duplicate = mysql_query("select count(*) as total from users where user_email='$usr_email' OR user_name='$user_name'") or die(mysql_error());
list($total) = mysql_fetch_row($rs_duplicate);
if ($total > 0)
$err[] = "ERROR - The username/email already exists. Please try again with different username and email.";
if(empty($err)) {
$sql_insert = "INSERT into `users`
              (`full_name`,`user_email`,`pwd`,`address`,`tel`,`fax`,`website`,`date`,`users_ip`,`activa tion_code`,`country`,`user_name`
            VALUES
            ('$data[full_name]','$usr_email','$sha1pass','$data[address]','$data[tel]','$data[fax]',' $data[web]'
            ,now(),'$user_ip','$activ_code','$data[country]','$user_name'
mysql_query($sql_insert,$link) or die("Insertion Failed:" . mysql_error());
$user_id = mysql_insert_id($link); 
$md5_id = md5($user_id);
mysql_query("update users set md5_id='$md5_id' where id='$user_id'");
//    echo "<h3>Thank You</h3> We received your submission.";
if($user_registration)  {
$a_link = "
*****ACTIVATION LINK*****\n
http://$host$path/activate.php?user=$md5_id&activ_code=$activ_code
} else {
$a_link =
"Your account is *PENDING APPROVAL* and will be soon activated the administrator.
$message =
"Hello \n
Thank you for registering with us. Here are your login details...\n
User ID: $user_name
Email: $usr_email \n
Passwd: $data[pwd] \n
$a_link
Thank You
Administrator
$host_upper
THIS IS AN AUTOMATED RESPONSE.
***DO NOT RESPOND TO THIS EMAIL****
    mail($usr_email, "Login Details", $message,
    "From: \"Member Registration\" <auto-reply@$host>\r\n" .
     "X-Mailer: PHP/" . phpversion());
  header("Location: thankyou.php"); 
  exit();
?>

Answer 1

I  have created a  log in system  . In that when the  user completes the  registration process an auto reply(auto-reply@domain)  will generate and  sent this to users email id regarding about the user  name and password  (Lo gin Details). After formal approval from the admin  the user will  get a user activation mail with log in link.
But , my problem is  these are work only for mail accounts from my  domain  only(test@domain). its not send any of above mentioned details  to other  mail services like gmail or yahoo etc.
i discussed this   with some others, they said its the problem with your mail function   configuration. but i didn't get any needful information as am a  beginner  in PHP scripting.
i have contacted this with my  hosting service they said its the  problem with  php mail () function  and use php mailer() instead mail().
please give me a solution for the same..
Here am ataching my code..
<?php
include 'dbc.php';
$err = array();
if($_POST['doRegister'] == 'Register')
foreach($_POST as $key => $value) {
    $data[$key] = filter($value);
if(empty($data['full_name']) || strlen($data['full_name']) < 4)
$err[] = "ERROR - Invalid name. Please enter atleast 3 or more characters for your name";
//header("Location: register.php?msg=$err");
//exit();
// Validate User Name
if (!isUserID($data['user_name'])) {
$err[] = "ERROR - Invalid user name. It can contain alphabet, number and underscore.";
//header("Location: register.php?msg=$err");
//exit();
// Validate Email
if(!isEmail($data['usr_email'])) {
$err[] = "ERROR - Invalid email address.";
//header("Location: register.php?msg=$err");
//exit();
// Check User Passwords
if (!checkPwd($data['pwd'],$data['pwd2'])) {
$err[] = "ERROR - Invalid Password or mismatch. Enter 5 chars or more";
//header("Location: register.php?msg=$err");
//exit();
$user_ip = $_SERVER['REMOTE_ADDR'];
// stores sha1 of password
$sha1pass = PwdHash($data['pwd']);
// Automatically collects the hostname or domain  like example.com)
$host  = $_SERVER['HTTP_HOST'];
$host_upper = strtoupper($host);
$path   = rtrim(dirname($_SERVER['PHP_SELF']), '/\\');
// Generates activation code simple 4 digit number
$activ_code = rand(1000,9999);
$usr_email = $data['usr_email'];
$user_name = $data['user_name'];
$rs_duplicate = mysql_query("select count(*) as total from users where user_email='$usr_email' OR user_name='$user_name'") or die(mysql_error());
list($total) = mysql_fetch_row($rs_duplicate);
if ($total > 0)
$err[] = "ERROR - The username/email already exists. Please try again with different username and email.";
if(empty($err)) {
$sql_insert = "INSERT into `users`
              (`full_name`,`user_email`,`pwd`,`address`,`tel`,`fax`,`website`,`date`,`users_ip`,`activa tion_code`,`country`,`user_name`
            VALUES
            ('$data[full_name]','$usr_email','$sha1pass','$data[address]','$data[tel]','$data[fax]',' $data[web]'
            ,now(),'$user_ip','$activ_code','$data[country]','$user_name'
mysql_query($sql_insert,$link) or die("Insertion Failed:" . mysql_error());
$user_id = mysql_insert_id($link); 
$md5_id = md5($user_id);
mysql_query("update users set md5_id='$md5_id' where id='$user_id'");
//    echo "<h3>Thank You</h3> We received your submission.";
if($user_registration)  {
$a_link = "
*****ACTIVATION LINK*****\n
http://$host$path/activate.php?user=$md5_id&activ_code=$activ_code
} else {
$a_link =
"Your account is *PENDING APPROVAL* and will be soon activated the administrator.
$message =
"Hello \n
Thank you for registering with us. Here are your login details...\n
User ID: $user_name
Email: $usr_email \n
Passwd: $data[pwd] \n
$a_link
Thank You
Administrator
$host_upper
THIS IS AN AUTOMATED RESPONSE.
***DO NOT RESPOND TO THIS EMAIL****
    mail($usr_email, "Login Details", $message,
    "From: \"Member Registration\" <auto-reply@$host>\r\n" .
     "X-Mailer: PHP/" . phpversion());
  header("Location: thankyou.php"); 
  exit();
?>