How do I check if an email address is valid without sending anything to it?

Question

I have a client with 5000 emails from an old list he has that he wants to promote his services to. He wants to know which emails on the list are still valid. I want to check them for him - without sending out 5K emails randomly and then being listed as a spammer or something. Ideas?

Answer 1

I think you'll have to send email to them. If you're worried about being flagged as a spammer you might try sending them in short bursts.

Answer 2

You'll have to email them at least once.

• Create a new email list. Send the old list an email with a link they need to click on to continue receiving messages (re-subscribe).
• Send them all an email and collect all reply-to bounces on a real email account, then purge those bounced emails from your main list.
• Send them all an HTML email, and one of the images is remotely hosted and requires a unique ID to request it that you set in each email. When your web server returns that image to their client, you can then consider that email as active. This is called a web bug, and will only work if the person automatically loads remote images in their client.

Answer 3

I think you need to send the emails to find out. Also, this is pretty much exactly what a spammer is, thus the reason for getting put on spammer lists. Sending in bursts will help you hide this fact though.

Answer 4

You can validate the email via SMTP without sending an actual email.

http://code.google.com/p/php-smtp-email-validation/

You could also send emails out, and check for bounces.

Answer 5

If it was me I would:

• Validate the email with a regular expression. Good example here.
• Ping or Whois the server to see if it is a valid domain. (This won't be 100% but should get one step closer)
• Send an email to the remaining addresses that are still in doubt.

Answer 6

bucabay's answer is the way forward. What a library like that essentially does is checking for existing DNS record for (mail) servers at specified domains (A, MX, or AAAA). After that, it do what's termed callback verification. That's where you connect to the mail server, tell it you want to send to a particular email address and see if they say OK.

For callback verification, you should note greylisting servers say OK to everything so there is no 100% guarantee possible without actually sending the emails out. Here's some code I used when I did this manually. It's a patch onto the email address parser from here.

    #
    # Email callback verification
    # Based on http://uk2.php.net/manual/en/function.getmxrr.php
    #

    if (strlen($bits['domain-literal'])){
        $records = array($bits['domain-literal']);
    }elseif (!getmxrr($bits['domain'], $mx_records, $mx_weight)){
        $records = array($bits['domain']);
    }else{
        $mxs = array();

        for ($i = 0; $i < count($mx_records); $i++){
            $mxs[$mx_records[$i]] = $mx_weight[$i];
        }

        asort($mxs);

        $records = array_keys($mxs);
    }

    $user_okay = false;
    for ($j = 0; $j < count($records) && !$user_okay; $j++){
        $fp = @fsockopen($records[$j], 25, $errno, $errstr, 2);
        if($fp){
            $ms_resp = "";

            $ms_resp .= send_command($fp, "HELO ******.com");
            $ms_resp .= send_command($fp, "MAIL FROM:<>");

            $rcpt_text = send_command($fp, "RCPT TO:<" . $email . ">");
            $ms_resp .= $rcpt_text;

            $ms_code = intval(substr($rcpt_text, 0, 3));
            if ($ms_code == 250 || $ms_code == 451){ // Accept all user account on greylisting server
                $user_okay = true;
            }

            $ms_resp .= send_command($fp, "QUIT");

            fclose($fp);
        }
    }

return $user_okay ? 1 : 0;