Check if email address exists front end with AJAX in a plugin

I am pulling my hair out trying to figure this out. I have followed the wordpress codex here for AJAX: http://codex.wordpress.org/AJAX_in_Plugins

All I keep returning is 0, I never get a 1. Any idea why? My code is as follows.

PHP:

add_action('wp_enqueue_scripts', 'live_validation' );
add_action('wp_ajax_validate_email', 'validate_email_input');
add_action('wp_ajax_nopriv_validate_email', 'validate_email_input');

function live_validation() {
    wp_enqueue_script( "validate_email", STRIPE_BASE_URL . 'inc/js/check-email.js', array( 'jquery' ) );
    wp_localize_script( "validate_email", "validateEmail", array( 'ajaxurl' => admin_url( 'admin-ajax.php' ) ) );
}

function validate_email_input() {
    $email = $_POST['email'];
    if ( email_exists($email) ) {
        echo 1; 
    } else {
        echo 0;
    }
    exit;
}

JS:

jQuery(document).ready(function($) {
    $('input[name=email]').keyup(function() {

        var input_value = $(this).val();

        $.post( validateEmail.ajaxurl, { action:'validate_email', email:input_value }, function(data) {
            $('.message').html(data);
        });
    });
});

HTML:

<div class="form-row">
    <label for="email"><?php _e('Email', 'oe'); ?></label>
    <input type="text" size="20" id="email" name="email" />
    <span class="message"></span>
</div>