To add to the conversation. There is a way to check if a mailbox exists using SMTP. It works on Gmail and several other servers. Python/PHP Code and explanatio

To add to the conversation. There is a way to check if a mailbox exists using SMTP. It works on Gmail and several other servers. Python/PHP Code and explanation is here http://www.webdigi.co.uk/blog/2009/how-to-check-if-an-email-address-exists-without-sending-an-email/ http://www.webdigi.co.uk/blog/2009/how-to-check-if-an-email-.... It was eye opening to understand the underlying SMTP protocol. There are some pitfalls too as mentioned in the article.