Email address validation is basically an impossible problem. In practice, the best strategy is:

    1. Check if email is wellformed, with an *extremely* lenient algorithm. Basically, do we have .*@.* ?

    2. Check that the domain is real, has an MX record, and the SMTP server in the MX record exists.

Ok, send mail! The only way to verify after that is to send an email with a verification link and complete the circle.

This is too strict: smtp tries A/AAAA/CNAME records if there is no MX.

Um... I hate to be so harsh but this is worthless. Checking a few of mine, all different domains, I got

Nemesis ESMTP Service not available 554-No SMTP service 554-IP address is black listed.

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This domain has a catch all email address so we are unable to determine if this email matches an account.

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Error: Invalid greeting from server - 554 No SMTPd here This email does not exist or cannot be found

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Error: Invalid greeting from server - 554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact XXXXXX This email does not exist or cannot be found

So... yeah... 100% error rate on something that says "the results should be relatively trustworthy." Relative to what? That success rate doesn't even crack the Mendoza Line.

Not working reliantly for me. Addresses that definitely exist are reported as not existing. No info on the checks used.

So, this could even just be a clever way to collect email addresses through a simple form.

Seems like a weird amount of work to collect emails when data leaks that contain millions of email addresses are widely available for download.

https://www.troyhunt.com/im-sorry-but-your-email-address-is-...

> So, this could even just be a clever way to collect email addresses through a simple form.

Exactly what I thought after pressing submit on my personal email (which was then reported as not existing). D'oh.

Heh, we all learned to disable VRFY 15-20 years ago.

This fails both ways on my tests.

Weird. I've disabled verify in Postfix on my mailserver, but it still can introspect email addresses:

disable_vrfy_command = yes

smtpd_delay_reject = yes

smtpd_helo_required = yes

Nope. Reports @example.[something] as existing, whereas info@[something else] - which regularly gets mail - as nonexisting. Both Type I and Type II failures, but confidently proclaims the verdict? Worse than useless: actively misleading.

It seems that some domains just kill the backend. For my work e-mail, and probably any other on the same domain, it returns HTTP 503 (service unavailable) after ~30s, and frontend is stuck at "Checking…" forever. Maybe our mailserver fights back the spammers somehow. :)

Also, it kinda sucks that i can't send the query with Enter key, and i bet it sucks much more for eg. visually impaired users. Is there any particular reason you used bunch of <div>s instead of good old <form>?

We use Cisco's cloud Ironport solution for email spam protection and I get the following when I check my email address: Error: Invalid greeting from server - XXXXXXXXX.iphmx.com 554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means.

A link to an "how it works" article would be nice.

Apparently tries to resolve MX, connect and try to poke around. Surprisingly, the last time that trick worked reliably was in the early 2000s, because guess who also used the same trick? Yup, spammers - so most servers will either a) deny existence for any address, or b) claim existence for every address. And that's before greylisting and SPF and whatnot.

The only way to be sure that the address actually exists is to send mail and have the recipient actively respond - however, if there's no response, it can mean any number of things: no such address, message got lost, message got spambinned, message got deleted by recipient, recipient couldn't be bothered to reply.

In other words, do not expect certainty from e-mail.

So it failed on my dynamical created email addresses

I have a system where i have double wildcard so [email protected] and your site fails

     Error: Spam detection in use
    This email does not exist or cannot be found

Worked for my person email attached to my domain. I agree with others in that explaining how this works would be very nice.

This is just a web-app using smtp to ask your mail server if it is willing to accept mail for your address, then leaving before actually passing any mail.

Am using namecheap's email service for my personal email and domain, this says it is an invalid email address

Yea this has not validated an email for me. Tried a few email clients and nothing said it was valid.

This is cool but yea some right up would be appreciated.