[slipdiegoknot]

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.