Using MX records to validate email addresses

Question

Scenario:
I have a contact form on my web app, it gets alot of spam.
I am validating the format of email addresses loosely i.e. ^.+@.+\\..+$

Answer 1

An MX Lookup is only part of the picture, if you want to ensure the email address is itself valid, then you need to attempt to send an email to that account.

The other possible scenario is, someone can be simply using hijacked email accounts from a compromised machine anyway. Of course, that is probably a little bit less likely to occur, but it still does.

There are email address validation libraries out there that do this, simply search for email validation.

All of this can be done asynchronously. I have this setup on my site in which case the email is saved in the database (for auditing purposes), a job queued, then when the job comes time to execute, any additional validation is performed at that point in time. It offloads the heavy lifting to another thread.

To the user, it appears as if the email was sent already, it was (it's in the database), and can be viewed internally, but the actual email won't get mailed out until that job executes which can be immediately or some set amount of time depending on the server load.

Walter

Answer 2

DNS lookups can be slow at times, depending on network traffic & congestion, so that's something to be aware of.

If I were in your shoes, I'd test it out and see how it goes. For a week or so, log all emails to a database or log file and include a field to indicate if it would be marked as spam or legitimate email. After the week is over, take a look at the results and see if it's performing as you would expect.

Taking this logging/testing approach gives you the flexibility to test it out and not worry about loosing customer emails.

I've gotten into the habit of adding an extra field to my forms that is hidden with CSS, if it's filled in I assume it's being submitted by a spam bot. I also make sure to use a name like "url" or "website_url" something that looks like a legitimate field name to a spam bot. Add a label for the field that says something like "Don't fill out this field" so if someone's browser doesn't render it correctly, they will know not to fill out the spam field. So far it's working very well for me.

Answer 3

function mxrecordValidate($email){
        list($user, $domain) = explode('@', $email);
        $arr= dns_get_record($domain,DNS_MX);
        if($arr[0]['host']==$domain&&!empty($arr[0]['target'])){
                return $arr[0]['target'];
        }
}
$email= 'user@radiffmail.com';

if(mxrecordValidate($email)) {
        echo('This MX records exists; I will accept this email as valid.');
}
else {
        echo('No MX record exists;  Invalid email.');
}

Answer 4

//The Code *https://davidwalsh.name/php-email-validator*  
function domain_exists($email, $record = 'MX'){
    list($user, $domain) = split('@', $email);
    return checkdnsrr($domain, $record);
}

if(domain_exists('user@davidwalsh.name')) {
    echo('This MX records exists; I will accept this email as valid.');
} else {
    echo('No MX record exists;  Invalid email.');
}

Answer 5

I see no harm doing a MX lookup with checkdnsrr() and I also don't see how false positives may appear. You don't need to escape the hostname, in fact you can use this technique and take it a little further by talking to the MTA and testing if the user exists at a given host (however this technique may and probably will get you some false positives in some hosts).