There are possibilities to validate if an email exists without sending an email. Your backend can open a socket the the email server of the email address and ask for the address. I'm not 100% sure if there could be a bouncer rule based on the content but it ensures at least that the given email address accepts emails from your server.
Here is an telnet example for this: webdigi.co.uk/blog/2009/how-to-che...
This probably only works for a few percent of mail servers these days, most will accept any RCPT TO address, wait for the DATA block and then black hole or reply with an error via a separate outbound message. Specifically done to permit more filtering (anti-spam etc.) and to /prevent/ these sorts of validity checks as they are popular with spammers (can't think why :))
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