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In the old days you'd simply connect to the domains SMTP and do either a VRFY or start sending an email and then aborting. Google doesn't allow this. However
by phillipseamore 6y ago
In the old days you'd simply connect to the domains SMTP and do either a VRFY or start sending an email and then aborting. Google doesn't allow this. However when signing up to Gmail you are notified if the username is taken, so they might be using something like that.