How is it possible to verify an email address without sending to it?

I always thought that since the VRFY smtp command is no-longer honoured by most servers (due to spammers abusing it) it is not possible to verify email addresses without actually sending something to them and seeing if the message bounces but I recently came across http://tools.email-checker.com/ which appears to work 100% of the time.

What technology/means could a tool like that site use in order to verify an email address? Could the same thing be done via telnet?

OK, it uses a bit of a trick. It connects to a mailserver, and sends sends a RCPT (mail receipt). If the mailbox email address is valid, the server accepts the message, and the sender is notified - thus validating the mailbox. However, the RCPT message has an invalid MsgID, and is denied. By this stage, however, it is too late.

I don’t know if this could be plugged easily without breaking SMTP - I’ll have to contact the qsmtp developers to see how this can be remedied. I don’t like it.

Si

Perhaps I’m paranoid but I’d be very wary of using that website - it looks to be a good way for spammers to get addresses.

I would recommend you to use Zerobounce.net.
The system works by reducing and eliminating invalid, abuse, complaint, inactive, and spam-trap email address. These are email addresses that will either bounce or contribute to ruining your sending reputation. They also provide IP address validation and verification of key recipient demographics and has the ability to add missing information on certain emails, such as the name, gender, and location of the owner.
They are the most secure email validation system you can find.
Here is the link where you can find them www.zerobounce.net