You would get an answer much faster in google....

One way is to set it up in the email, for example a new user registers and an email goes to u and a cc or bcc to welcome@yoursite.com, u set up welcome@yoursite.com to auto reply all incoming emails and set some text as default. Set the email to delete all mails after some time so it doesnt get full.

Or.. u can use some type of script to fit ure needs.. for example:

PHP Code: 
<?php
$contact_name 
$_POST['name'];
$contact_email $_POST['email'];
$contact_subject $_POST['subject'];
$contact_message $_POST['message'];
 
if( 
$contact_name == true )
{
 
$sender $contact_email;
 
$receiver "myemailhost.com";
 
$client_ip $_SERVER['REMOTE_ADDR'];
 
$email_body "Name: $contact_name \nEmail: $sender \nSubject: $contact_subject \nMessage: $contact_message \nIP: $client_ip \nFrom Industrial Placement Website";  
 
$extra "From: $sender\r\n" "Reply-To: $sender \r\n" "X-Mailer: PHP/" phpversion();
 if( 
mail$receiver"Flash Contact Form - $subject"$email_body$extra ) ) 
 {
  echo 
"success=yes";
 }
 else
 {
  echo 
"success=no";
 }
}
// The Reply
$email $_POST['email']; 
$header "myemail.com";
$subject 'Submission'// change subject
$message 'This is a confirmation that we have recieved your message, and we will be in contact with you soon regarding your enquiry. 
Thanks for your interest.
Bailey
'
// change text
mail($email$subject$message'From: '.$receiver.''); // send another one out
?>
You should really read this before bro: http://www.tectite.com/fmhowto/autorespond.php

Understand the spamming that can come out of ure site by doing this wrong. A simple ?THANK YOU? message can do the job...