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The best and most recommended ways to verify an email address without sending an email are: Email verifier tools: Use an email verification service to check if the given address is valid or not. Just google 'Email Verifier,' and many free and paid options will come up.
The simplest way to verify the validity of an email address is to send a test email. If the email hard bounces, i.e. there will be no further attempt to deliver a message, the recipient does not exist.
Email Checker is a simple little tool for verifying an email address. It's free and quite easy to use. Just enter the email address and hit the check button. It tells you whether the email id is real or fake.
How do you know if an email address exists or not? The easy option would be that you send a dummy mail to that email address, wait for an hour or so and if your message bounces, it is very likely that the particular email address does not exist.
There are two methods you can sometimes use to determine if a recipient actually exists:
You can connect to the server, and issue a VRFY command. Very few servers support this command, but it is intended for exactly this. If the server responds with a 2.0.0 DSN, the user exists.
VRFY user
You can issue a RCPT, and see if the mail is rejected.
MAIL FROM:<>
RCPT TO:<user@domain>
If the user doesn't exist, you'll get a 5.1.1 DSN. However, just because the email is not rejected, does not mean the user exists. Some server will silently discard requests like this to prevent enumeration of their users. Other servers cannot verify the user and have to accept the message regardless.
There is also an antispam technique called greylisting, which will cause the server to reject the address initially, expecting a real SMTP server would attempt a re-delivery some time later. This will mess up attempts to validate the address.
Honestly, if you're attempting to validate an address the best approach is to use a simple regex to block obviously invalid addresses, and then send an actual email with a link back to your system that will validate the email was received. This also ensures that they user entered their actual email, not a slight typo that happens to belong to somebody else.
Other answers here discuss the various problems with trying to do this. I thought I'd show how you might try this in case you wanted to learn by doing it yourself.
You can connect to an mail server via telnet to ask whether an email address exists. Here's an example of testing an email address for stackoverflow.com:
C:\>nslookup -q=mx stackoverflow.com Non-authoritative answer: stackoverflow.com MX preference = 40, mail exchanger = STACKOVERFLOW.COM.S9B2.PSMTP.com stackoverflow.com MX preference = 10, mail exchanger = STACKOVERFLOW.COM.S9A1.PSMTP.com stackoverflow.com MX preference = 20, mail exchanger = STACKOVERFLOW.COM.S9A2.PSMTP.com stackoverflow.com MX preference = 30, mail exchanger = STACKOVERFLOW.COM.S9B1.PSMTP.com C:\>telnet STACKOVERFLOW.COM.S9A1.PSMTP.com 25 220 Postini ESMTP 213 y6_35_0c4 ready. CA Business and Professions Code Section 17538.45 forbids use of this system for unsolicited electronic mail advertisements. helo hi 250 Postini says hello back mail from: <[email protected]> 250 Ok rcpt to: <[email protected]> 550-5.1.1 The email account that you tried to reach does not exist. Please try 550-5.1.1 double-checking the recipient's email address for typos or 550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1 http://mail.google.com/support/bin/answer.py?answer=6596 w41si3198459wfd.71
Lines prefixed with numeric codes are responses from the SMTP server. I added some blank lines to make it more readable.
Many mail servers will not return this information as a means to prevent against email address harvesting by spammers, so you cannot rely on this technique. However you may have some success at cleaning out some obviously bad email addresses by detecting invalid mail servers, or having recipient addresses rejected as above.
Note too that mail servers may blacklist you if you make too many requests of them.
In PHP I believe you can use fsockopen, fwrite and fread to perform the above steps programmatically:
$smtp_server = fsockopen("STACKOVERFLOW.COM.S9A1.PSMTP.com", 25, $errno, $errstr, 30);
fwrite($smtp_server, "helo hi\r\n");
fwrite($smtp_server, "mail from: <[email protected]>\r\n");
fwrite($smtp_server, "rcpt to: <[email protected]>\r\n");
The general answer is that you can not check if an email address exists event if you send an email to it: it could just go into a black hole.
That being said the method described there is quite effective. It is used in production code in ZoneCheck except that it uses RSET instead of QUIT.
Where user interaction with his mailbox is not overcostly many sites actually test that the mail arrive somewhere by sending a secret number that must be sent back to the emitter (either by going to a secret URL or sending back this secret number by email). Most mailing lists work like that.
This will fail (amongst other cases) when the target mailserver uses greylisting.
Greylisting: SMTP server refuses delivery the first time a previously unknown client connects, allows next time(s); this keeps some percentage of spambots out, while allowing legitimate use - as it is expected that a legitimate mail sender will retry, which is what normal mail transfer agents will do.
However, if your code only checks on the server once, a server with greylisting will deny delivery (as your client is connecting for the first time); unless you check again in a little while, you may be incorrectly rejecting valid e-mail addresses.
Not really.....Some server may not check the "rcpt to:"
http://www.freesoft.org/CIE/RFC/1123/92.htm
Doing so is security risk.....
If the server do, you can write a bot to discovery every address on the server....
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