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SMTP's the API for checking if an address exists at any mail provider. Start sending a mail, if the server doesn't tell you there's no such mailbox right then, you can abandon the connection without sending a message through. No CAPTCHAs there either.

Many mail servers delay account checking for that very reason.

They take in all mail - instead of telling the sending MTA that the account doesn't exist the message is accepted and sent to /dev/null. A bounce message may be generated but the automated MTA won't see that as the message will be carrying invalid mail sender information.

Many mail relays, especially spam appliances like Barracuda, will always give an 250 OK to the email address provided to prevent exactly this trick. They then toss as spam or bounce once they get the message.

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