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When getting a delivery status notification (Failure) this snippet is included into the message:

Technical details of permanent failure: Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 5.1.1 User unknown (state 13).

So this means I guess that before sending the mail google made a query if the target mail server will accept it. How could I reproduce this query?

In other words, I would like to manually query a mail server if an address exits or not. How do I do this?

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Why do you want to do this? It doesn't strike me as a very useful or productive thing to do. – joeqwerty Jan 9 '13 at 23:53
@joeqwerty It's honestly a fair question for someone who doesn't have a background in MTA software. It just so happens that not having that background also means the same person won't immediately recognize that such functionality would enable spammers. – Andrew B Jan 10 '13 at 0:20
In the past there used to be a SMTP verbs VRFY, and EXPN. These have been disabled, on almost every mail server on the Internet. The only way to test if an account exists, is by trying to send a message. – Zoredache Jan 10 '13 at 0:24
up vote 8 down vote accepted

So this means I guess that before sending the mail google made a query if the target mail server will accept it

No. It means when google tried to deliver it, the server responded with user unknown.

More in details:

550 550 5.1.1


READ THE DOCUMENTATION (smtp protocol) there actually IS a command to ask whether a mailbox exist.

Here is the bad news: most servers will NEVER answer correctly but just say yes and throw the email away. Because of Spam.

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