How to check if an email address exists without sending an email?

We have all been doing email address validation for a very long time to make sure that the email is correctly formatted. This is to avoid users entering wrongly formatted email address but still they can accidentally give us a wrong email address.

Example of a correctly formatted email address but still wrong:

mailbox.does.not.exist@domain.tld [VALID format but does not exist]

Above case specifically happens when you take important customer email on phone and you type in the wrong email. So is there a QUICK solution to really check the email without sending a test message to the user? Yes.

How to test if the email address actually exists

To check if user entered email mailbox.does.not.exist@domain.tld really exists go through the following in command prompt on Windows / Terminal on Mac.

Step 1

Find mail exchanger or mail server of domain.tld

nslookup -q=mx domain.tld

Non-authoritative answer:
domain.tld mail exchanger = 0 mx2.sub3.homie.mail.dreamhost.com.
domain.tld mail exchanger = 0 mx1.sub3.homie.mail.dreamhost.com.

Step 2

Now we know the mail server address so let us connect to it. You can connect to one of the exchanger addresses in the response from Step 1.

telnet mx2.sub3.homie.mail.dreamhost.com 25

Connected to mx2.sub3.homie.mail.dreamhost.com.
Escape character is ‘^]’.
220 homiemail-mx7.g.dreamhost.com ESMTP
helo hi

250 homiemail-mx8.g.dreamhost.com
mail from: <youremail@gmail.com>

250 2.1.0 Ok
rcpt to: <mailbox.does.not.exist@domain.tld>

550 5.1.1 <mailbox.does.not.exist@domain.tld>: Recipient address rejected: User unknown in virtual alias table
quit

221 2.0.0 Bye

NOTES:

  1. the 550 response indicates that the email address is not valid and you have caught a valid but wrong email address. This code can be on the server and called on AJAX when user tabs out of the email field.  The entire check will take less than 2 seconds to run and you can make sure that the email is correct.
  2. If email was present the server will respond with a 250 instead of 550
  3. There are certain servers with a CATCH ALL email and this means all email address are accepted as valid on their servers (RARE but some servers do have this setting).
  4. Please do not use this method to continuously to check for availability of gmail / yahoo / msn accounts etc as this may cause your IP to be added to a blacklist.
  5. This is to supplement the standard email address javascript validation.