Validate email really exists

One of the reasons it is so difficult to check if an email exists (without sending an email) is that spammers could then brute force every valid email for a domain.

Yes! correct, many email provider companies block this validation such as gmail and others don't thus making any validation method unrealiable. I gave up on this one.

For example here http://verify-email.org/ it really checks that gmail accounts exist. I guess it can be done after all.