Validate email with smtp

This function validate a mail address in a smart way: if the address to validate has a correct syntax (checked with regular expression), it tries to connect to the mx server of the address. This means that the program have to use the smtp protocol to talk with each mx server registered in the dns for that domain address.
When a connection with an mx server is established, the program presents himself with the $probe_address (if no probe_address is specified it will use the SERVER_ADMIN setting of the php.ini file) and asks to send a mail message to the address to validate. If the server responds that the destination mailbox is ok, it stops and return ok value, else it gives you the bad answer of the mail server.
Sometimes this process fails due to timeouts, or gives a bad answer because some mail servers are configured to respond always: “yes this destination mail exists!”… So… you should trust this code not at 100%.
The best way to check if a mail address really exists is double opt in. But this function is cool, suppose to use it in an ajax form. :-)
Here is a working demo of Smtp Mail Validation function.

This code is included into the Mini Bots Php Class.

function validateEmailSmtp($email, $probe_address="", $debug=false) {
	# --------------------------------
	# function to validate email address
	# through a smtp connection with the
	# mail server.
	# by Giulio Pons
	# https://www.barattalo.it
	# --------------------------------
	$output = "";
	# --------------------------------
	# Check syntax with regular expression
	# --------------------------------
	if (!$probe_address) $probe_address = $_SERVER["SERVER_ADMIN"];
	if (preg_match('/^([a-zA-Z0-9\._\+-]+)\@((\[?)[a-zA-Z0-9\-\.]+\.([a-zA-Z]{2,7}|[0-9]{1,3})(\]?))$/', $email, $matches)) {
		$user = $matches[1];
		$domain = $matches[2];
		# --------------------------------
		# Check availability of DNS MX records
		# --------------------------------
		if (function_exists('checkdnsrr')) {
			# --------------------------------
			# Construct array of available mailservers
			# --------------------------------
			if(getmxrr($domain, $mxhosts, $mxweight)) {
				for($i=0;$i<count($mxhosts);$i++){
					$mxs[$mxhosts[$i]] = $mxweight[$i];
				}
				asort($mxs);
				$mailers = array_keys($mxs);
			} elseif(checkdnsrr($domain, 'A')) {
				$mailers[0] = gethostbyname($domain);
			} else {
				$mailers=array();
			}
			$total = count($mailers);
			# --------------------------------
			# Query each mailserver
			# --------------------------------
			if($total > 0) {
				# --------------------------------
				# Check if mailers accept mail
				# --------------------------------
				for($n=0; $n < $total; $n++) {
					# --------------------------------
					# Check if socket can be opened
					# --------------------------------
					if($debug) { $output .= "Checking server $mailers[$n]...\n";}
					$connect_timeout = 2;
					$errno = 0;
					$errstr = 0;
					# --------------------------------
					# controllo probe address
					# --------------------------------
					if (preg_match('/^([a-zA-Z0-9\._\+-]+)\@((\[?)[a-zA-Z0-9\-\.]+\.([a-zA-Z]{2,7}|[0-9]{1,3})(\]?))$/', $probe_address,$fakematches)) {
						$probe_domain = str_replace("@","",strstr($probe_address, '@'));

						# --------------------------------
						# Try to open up socket
						# --------------------------------
						if($sock = @fsockopen($mailers[$n], 25, $errno , $errstr, $connect_timeout)) {
							$response = fgets($sock);
							if($debug) {$output .= "Opening up socket to $mailers[$n]... Success!\n";}
							stream_set_timeout($sock, 5);
							$meta = stream_get_meta_data($sock);
							if($debug) { $output .= "$mailers[$n] replied: $response\n";}
							# --------------------------------
							# Be sure to set this correctly!
							# --------------------------------
							$cmds = array(
								"HELO $probe_domain",
								"MAIL FROM: <$probe_address>",
								"RCPT TO: <$email>",
								"QUIT",
							);
							# --------------------------------
							# Hard error on connect -> break out
							# --------------------------------
							if(!$meta['timed_out'] && !preg_match('/^2\d\d[ -]/', $response)) {
								$codice = trim(substr(trim($response),0,3));
								if ($codice=="421") {
									//421 #4.4.5 Too many connections to this host.
									$error = $response;
									break;
								} else {
									if($response=="" || $codice=="") {
										//c'è stato un errore ma la situazione è poco chiara
										$codice = "0";
									}
									$error = "Error: $mailers[$n] said: $response\n";
									break;
								}
								break;
							}
							foreach($cmds as $cmd) {
								$before = microtime(true);
								fputs($sock, "$cmd\r\n");
								$response = fgets($sock, 4096);
								$t = 1000*(microtime(true)-$before);
								if($debug) {$output .= "$cmd\n$response" . "(" . sprintf('%.2f', $t) . " ms)\n";}
								if(!$meta['timed_out'] && preg_match('/^5\d\d[ -]/', $response)) {
									$codice = trim(substr(trim($response),0,3));
									if ($codice<>"552") {
										$error = "Unverified address: $mailers[$n] said: $response";
										break 2;
									} else {
										$error = $response;
										break 2;
									}
									# --------------------------------
									// il 554 e il 552 sono quota
									// 554 Recipient address rejected: mailbox overquota
									// 552 RCPT TO: Mailbox disk quota exceeded
									# --------------------------------
								}
							}
							fclose($sock);
							if($debug) { $output .= "Succesful communication with $mailers[$n], no hard errors, assuming OK\n";}
							break;
						} elseif($n == $total-1) {
							$error = "None of the mailservers listed for $domain could be contacted";
							$codice = "0";
						}
					} else {
						$error = "Il probe_address non è una mail valida.";
					}
				}
			} elseif($total <= 0) {
				$error = "No usable DNS records found for domain '$domain'";
			}
		}
	} else {
		$error = 'Address syntax not correct';
	}
	if($debug) {
		print nl2br(htmlentities($output));
	}
	if(!isset($codice)) {$codice="n.a.";}
	if(isset($error)) return array($error,$codice); else return true;
}