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

Returned Email

When an email message bounces back there is always the question: is it us? is it them? is it just a typo?  You may receive a bounce-back email, but trying to decypher can be akin to translating ancient greek.

Here is a quick & simple check to get to the heart of the problem

To check if user entered email mailbox.does.not.exist@reddit.com really exists go through the following in command prompt. Click start and on the run line type in the characters CMD to get to a command prompt (looks like an old DOS screen).

First - Find mail exchanger of reddit.com (after each line press the enter key):

COMMAND:
nslookup
set q=mx
reddit.com
RESPONSE:
reddit.com      MX preference = 10, mail exchanger = mail.reddit.com
mail.reddit.com internet address = 208.96.53.70

Second - Connect to mail server mail.reddit.com

COMMAND:
telnet inbound.onsitelogic.com.netsolmail.net 25
RESPONSE:
220 mail.reddit.com ESMTP Postfix NO UCE NO UEMA  C=US L=CA Unsolicated electronic mail advertisements strictly prohibited, subject to fine under CA law CBPC 17538.45.  This electronic mail service provider’s equipment is located in the State of California.  See http://www.reddit.com/static/inbound-email-policy.html for more information.

COMMAND:
helo hi
RESPONSE:
250 mail.reddit.com

COMMAND:
mail from: <youremail@yourcompany.com>
RESPONSE:
250 2.1.0 Ok

COMMAND:
rcpt to: <mailbox.does.not.exist@reddit.com>
RESPONSE:
550 5.1.1 <mailbox.does.not.exist@reddit.com>: Recipient address rejected: User unknown in local recipient table

COMMAND:
quit
RESPONSE:
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.

(thanks to www.webdigi.com.uk and php-manual for documenting the steps)