<?php
function validEmail($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 $isValid;
}
if($_POST) {
	//$email = 'kathi.jones@youthhome.org'; 
	echo date('Y-m-d H:i:s')."<br/>"; 
	var_dump(validEmail($_POST['email'])); 
}

?>
<form action="<?=$_SERVER['SCRIPT_NAME'];?>" method="post">
	<input type="text" name="email" value="<?=($_POST['email']) ? $_POST['email'] : null; ?>"/>
	<input type="submit" value="Check Email"/>
</form>