Tester addresse mail en PHP

function validate_email($email){
   $mailparts=explode("@",$email);
   $hostname = $mailparts[1];

   // validate email address syntax
   $exp = "^[a-z\'0-9]+([._-][a-z\'0-9]+)*@([a-z0-9]+([._-][a-z0-9]+))+$";
   $b_valid_syntax=eregi($exp, $email);

   // get mx addresses by getmxrr
   $b_mx_avail=getmxrr( $hostname, $mx_records, $mx_weight );
   $b_server_found=0;

   if($b_valid_syntax && $b_mx_avail){
     // copy mx records and weight into array $mxs
     $mxs=array();


     for($i=0;$i<count($mx_records);$i++){
     $em1 = array_pop($mx_weight);
     $em2 = array_pop($mx_records);
       $mxs[$em1]=$em2;
     }

     // sort array mxs to get servers with highest prio
     ksort ($mxs, SORT_NUMERIC );
     reset ($mxs);

     while (list ($mx_weight, $mx_host) = each ($mxs) ) {
       if($b_server_found == 0){

         //try connection on port 25
         $fp = @fsockopen($mx_host,25, $errno, $errstr, 2);
         if($fp){
           $ms_resp="";
           // say HELO to mailserver
           $ms_resp.=send_command($fp, "HELO microsoft.com");

           // initialize sending mail
           $ms_resp.=send_command($fp, "MAIL FROM:<support@microsoft.com>");

           // try receipent address, will return 250 when ok..
           $rcpt_text=send_command($fp, "RCPT TO:<".$email.">");
           $ms_resp.=$rcpt_text;
          
           if(substr( $rcpt_text, 0, 3) == "250")
             $b_server_found=1;

           // quit mail server connection
           $ms_resp.=send_command($fp, "QUIT");

         fclose($fp);

         }

       }
   }
  }
  return $b_server_found;
}

function send_command($fp, $out){

  fwrite($fp, $out . "\r\n");
  return get_data($fp);
}

function get_data($fp){
  $s="";
  stream_set_timeout($fp, 2);

  for($i=0;$i<2;$i++)
   $s.=fgets($fp, 1024);

  return $s;
}

// support windows platforms
if (!function_exists ('getmxrr') ) {
  function getmxrr($hostname, &$mxhosts, &$mxweight) {
   if (!is_array ($mxhosts) ) {
     $mxhosts = array ();
   }

   if (!empty ($hostname) ) {
     $output = "";
     @exec ("nslookup.exe -type=MX $hostname.", $output);
     $imx=-1;

     foreach ($output as $line) {
       $imx++;
       $parts = "";
       if (preg_match ("/^$hostname\tMX preference = ([0-9]+), mail exchanger = (.*)$/", $line, $parts) ) {
         $mxweight[$imx] = $parts[1];
         $mxhosts[$imx] = $parts[2];
       }
     }
     return ($imx!=-1);
   }
   return false;
  }
}

Function ControleContact($email,$nomorganisation,$service,$fonction){
	$msg="";
  $email = trim($email);  
	if(empty($nomorganisation)):
		$msg .= "Le champs <B>Nom de l'organisation</B> est obligatoire<BR>";
	endif;
	if(!is_string($nomorganisation))
		$msg .= "Le champs <B>Nom de l'organisation</B> doit être une chaine de caractére<BR>";
	if(empty($service))
		$msg .= "Au moins une case <B>service</B> doit être cochée<BR>";
	if(empty($fonction))
		$msg .= "Une <B>fonction</B> doit être séléctionnée<BR>";
	if(empty($email))
    $msg .="Le champ <B>email</B> est obligatoire<BR>";
  elseif(!ereg("^(.+)@(.+)\\.(.+)$",$email))
    $msg .="Le champ <B>email</B> est obligatoire 22<BR>";
  elseif(validate_email($email) == 0) {
    echo 'Invalid email address!';
    $msg .='Invalid email address!';
    }
  else {
     echo 'Email address is valid';
    }    
	return ($msg);
}

// support windows platforms
if (!function_exists ('getmxrr') ) {
  function getmxrr($hostname, &$mxhosts) {
   if (!is_array ($mxhosts) ) {
     $mxhosts = array ();
   }

   if (!empty ($hostname) ) {
   echo "$hostname n'est pas vide !!<br>";
     $output = "";
     @exec ("nslookup.exe -type=MX $hostname.", $output);
     $imx=-1;

     foreach ($output as $line) {
       $imx++;
       $parts = "";
       if (preg_match ("/^$hostname\tMX preference = ([0-9]+), mail exchanger = (.*)$/", $line, $parts) ) {
//         $mxweight[$imx] = $parts[1];
         $mxhosts[$imx] = $parts[2];
       }
     }
     return ($imx!=-1);
   }
   return false;
  }
}

<?php
  /**
   * Check validity of e-mail
   *
   * @param string email
   * @return boolean
   */
function checkemail($email)
{
  define('TIMEOUT', '5');
  define('SMTP_PORT', '25');
  // see http://www.devshed.com/c/a/PHP/Email-Address-Verification-with-PHP/2/
  // checks proper syntax

  if (!preg_match("/^([a-zA-Z0-9])+([a-zA-Z0-9\._-])*@([a-zA-Z0-9_-])+([a-zA-Z0-9\._-]+)+$/" , $email))
    {
      //printf('%s: %d<br />', __FILE__,  __LINE__);
      return false;
    }

  // gets domain name
  list($username,$domain)=split('@',$email);
  // checks for if MX records in the DNS
  $mxhosts = array();
  if(!getmxrr($domain, $mxhosts)) {
    // no mx records, ok to check domain
    if (!@fsockopen($domain, SMTP_PORT, $errno, $errstr, TIMEOUT))
      {
.....

function checkEmail($email) {
 // checks proper syntax
 if(preg_match("/^( [a-zA-Z0-9] )+( [a-zA-Z0-9\._-] )*@( [a-zA-Z0-9_-] )+( [a-zA-Z0-9\._-] +)+$/" , $email)) {
  // gets domain name
  list($username,$domain)=split('@',$email);
  // checks for if MX records in the DNS
  if(!customCheckDnsrr($domain)) {
   return false;
  }
  // attempts a socket connection to mail server
  if(!fsockopen($domain,25,$errno,$errstr,30)) {
   return false;
  }
  return true;
 }
 return false;
}

function customCheckDnsrr($host,$recType='') {
 if(!empty($host)) {
  if($recType=='') $recType="MX";
  exec("nslookup -type=$recType $host",$output);
  foreach($output as $line) {
   if(preg_match("/^$host/", $line)) {
    return true;
   }
  }
  return false;
 }
 return false;
}

Function ControleContact($email,$nomorganisation,$service,$fonction){
	$msg="";
	if(empty($nomorganisation)):
		$msg .= "Le champs <B>Nom de l'organisation</B> est obligatoire<BR>";
	endif;
	if(!is_string($nomorganisation))
		$msg .= "Le champs <B>Nom de l'organisation</B> doit être une chaine de caractére<BR>";
	if(empty($service))
		$msg .= "Au moins une case <B>service</B> doit être cochée<BR>";
	if(empty($fonction))
		$msg .= "Une <B>fonction</B> doit être séléctionnée<BR>";
	if(empty($email))
    $msg .="Le champ <B>email</B> est obligatoire<BR>";
    else
/*
** C'est là que commence réellement l'appel
*/
$email = trim($email);  
if(!checkEmail($email)) { 
echo 'Invalid email address!';
$msg .='Invalid email address!';
}
else {
 echo 'Email address is valid';
}    
/*
** C'est là que fini réellement l'appel
*/

	return ($msg);
}

Le fait qu'un serveur de ne réponde pas concernant la validité du mail constitue une protection contre le spam il me semble.