gistfile1.php

<?php
/**
Validate an email address.
Provide email address (raw input)
Returns true if the email address has the email 
address format and the domain exists.
*/

function check_email_valid($email){
   $isValid = true;
   $atIndex = strrpos($email, "@");
   if (is_bool($atIndex) && !$atIndex)
   {
      $isValid = false;
   }
   else 
   {
      $domain = substr($email, $atIndex+1);
      $local = substr($email, 0, $atIndex);
      $localLen = strlen($local);
      $domainLen = strlen($domain);

      if ($localLen < 1 || $localLen > 64){
         // local part length exceeded
         $isValid = false;
      }else if ($domainLen < 1 || $domainLen > 255){
         // domain part length exceeded
         $isValid = false;
      }else if ($local[0] == '.' || $local[$localLen-1] == '.'){
         // local part starts or ends with '.'
         $isValid = false;
      }else if (preg_match('/\\.\\./', $local)){
         // local part has two consecutive dots
         $isValid = false;
      }else if (!preg_match('/^[A-Za-z0-9\\-\\.]+$/', $domain)){
         // character not valid in domain part
         $isValid = false;
      }else if (preg_match('/\\.\\./', $domain)){
         // domain part has two consecutive dots
         $isValid = false;
      }else if(!preg_match('/^(\\\\.|[A-Za-z0-9!#%&`_=\\/$\'*+?^{}|~.-])+$/', str_replace("\\\\","",$local))){
         // character not valid in local part unless 
         // local part is quoted
         if (!preg_match('/^"(\\\\"|[^"])+"$/',str_replace("\\\\","",$local))){
            $isValid = false;
         }
      }
      if($isValid && !(checkdnsrr($domain,"MX") || checkdnsrr($domain,"A"))){
         // domain not found in DNS
         $isValid = false;
      }
   }
   return $isValid;
}

?>