Hello,
What service can I use or script that checks to see if the user's email is authentic? For example, if a user tries to input a fake email or an email that does not exist. Then it will instantly check the authenticity.
It's impossible to do reliably. More info.
I would first validate the email with regex on the client side (javascript). If the regular expressions are too complicated, there are a plethora of jQuery form validation plugins that validate email addresses.
After that first check is passed, validate that the domain has MX DNS records configured on the server side. More info.
The last part can't be relied upon to be done in real time, but you'd need to send an email, and invalidate the user if it bounces.
Holy crap that sounds complicated. Services like aweber doesn't have this?