function EmailValidation($email)
 { 
    $email = htmlspecialchars(stripslashes(strip_tags($email))); //parse unnecessary characters to prevent exploits
 
    if ( eregi ( '[a-z||0-9]@[a-z||0-9].[a-z]', $email ) ) 
	{ //checks to make sure the email address is in a valid format
    $domain = explode( "@", $email ); //get the domain name
        if ( @fsockopen ($domain[1],80,$errno,$errstr,3)) 
		{
            //if the connection can be established, the email address is probably valid
            return true;
        } else 
		{
            return false; //if a connection cannot be established return false
        }
    return false; //if email address is an invalid format return false
  }
}

RCPT TO: mail@otherdomain.ext

 
$email = 'user@example.com';
list($user, $domain) = explode('@', $email);
 
$port = 25; // default smtp port
 
$sock = fsockopen($domain, $port);
if ($sock) {
 
fputs($sock, 'HELO mydomain.com');
$reply = fgets($sock); // not interesting
 
fputs($sock, 'MAIL FROM: user@mydomain.com');
$reply = fgets($sock); // not interesting
 
fputs($sock, 'RCPT TO: '.$email);
$reply = fgets($sock); // interesting
 
list($code, $msg) = explode(' ', $reply);
 
if ($code == '250') {
  // you received 250 so the email address is accepted
} else {
 // something went wrong, the email will most likely bounce
}
 
fclose($sock);
 
}

/**
 * Validate an Email Address Via SMTP
 * This queries the SMTP server to see if the email address is accepted.
 * @copyright http://creativecommons.org/licenses/by/2.0/ - Please keep this comment intact
 * @author gabe@fijiwebdesign.com
 * @version 0.1a
 */
class SMTP_validateEmail {
 
	var $sock;
 
	var $user;
	var $domain;
	var $port = 25;
	var $max_conn_time = 30;
	var $max_read_time = 5;
 
	var $from_user = 'user';
	var $from_domain = 'localhost';
 
	var $debug = false;
 
	function SMTP_validateEmail($email = false, $sender = false) {
		if ($email) {
			$this->setEmail($email);
		}
		if ($sender) {
			$this->setSenderEmail($sender);
		}
	}
 
	function setEmail($email) {
		list($user, $domain) = explode('@', $email);
		$this->user = $user;
		$this->domain = $domain;
	}
 
	function setSenderEmail($email) {
		list($user, $domain) = explode('@', $email);
		$this->from_user = $user;
		$this->from_domain = $domain;
	}
 
	/**
	* Validate an Email Address
	* @param String $email Email to validate (recipient email)
	* @param String $sender Senders Email
	*/
	function validate($email = false, $sender = false) {
		if ($email) {
			$this->setEmail($email);
		}
		if ($sender) {
			$this->setSenderEmail($sender);
		}
		// retrieve SMTP Server via MX query on domain
		$result = getmxrr($this->domain, $hosts);
		// last fallback is the original domain
		array_push($hosts, $this->domain);
 
		$this->debug(print_r($hosts,1 ));
 
		$timeout = $this->max_conn_time/count($hosts);
 
		// try each host
		foreach($hosts as $host) {
			// connect to SMTP server
			$this->debug("try $host:$this->port\n");
			if ($this->sock = fsockopen($host, $this->port, $errno, $errstr, (float) $timeout)) {
				socket_set_blocking($this->sock, false);
				break;
			}
		}
 
		// did we get a TCP socket
		if ($this->sock) {
			// say helo
			$this->send("HELO ".$this->from_domain);
			// tell of sender
			$this->send("MAIL FROM: <".$this->from_user.'@'.$this->from_domain.">");
			// ask of recepient
			$reply = $this->send("RCPT TO: <".$this->user.'@'.$this->domain.">");
			// quit
			$this->send("quit");
			// close socket
			fclose($this->sock);
			// get code and msg from response
			list($code, $msg) = explode(' ', $reply);
 
			if ($code == '250') {
				// you received 250 so the email address was accepted
				return true;
			}
 
		}
 
		return false;
	}
 
 
	function send($msg) {
		fwrite($this->sock, $msg."\n");
		$resp = '';
		$start = $this->microtime_float();
		while(1) {
			$reply = fread($this->sock, 2082);
			$resp .= $reply;
			if (($resp != '' && trim($reply) == '') 
			|| ($this->microtime_float() - $start > $this->max_read_time )) {
				break;
			}
		}
 
		$this->debug(">>>\n$msg\n");
		$this->debug("<<<\n$resp");
 
		return $resp;
	}
 
	/**
	 * Simple function to replicate PHP 5 behaviour. http://php.net/microtime
	 */
	function microtime_float() {
		list($usec, $sec) = explode(" ", microtime());
		return ((float)$usec + (float)$sec);
	}
 
	function debug($str) {
		if ($this->debug) {
			echo $str;
		}
	}
 
 
}

// the email to validate
$email = 'joe@gmail.com';
// an optional sender
$sender = 'user@example.com';
// instantiate the class
$SMTP_Valid = new SMTP_validateEmail();
// turn debugging on to view the SMTP session
$SMTP_Valid->debug = true;
// do the validation
$result = $SMTP_Valid->validate($email, $sender);
// view results
var_dump($result);
echo $email.' is '.($result ? 'valid' : 'invalid')."\n";
 
// send email? 
if ($result) {
  //mail(...);
}