PHP Snippet: isValidEmail­Address($email)

<?php

function isValidEmailAddress($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;
}