Email Verification for WordPress - Block Fake Registrations at Sign-Up

Add this to your theme's functions.php or a custom plugin. Validates every registration before it hits the database.

add_filter( 'registration_errors', function( $errors, $sanitized_user_login, $user_email ) {
    $api_key = 'YOUR_BOUNCEZERO_API_KEY';

    $response = wp_remote_post( 'https://api.bouncezero.io/api/v1/verify', [
        'headers' => [
            'Content-Type'  => 'application/json',
            'Authorization' => 'Bearer ' . $api_key,
        ],
        'body'    => json_encode( [ 'email' => $user_email ] ),
        'timeout' => 5,
    ] );

    if ( ! is_wp_error( $response ) ) {
        $data = json_decode( wp_remote_retrieve_body( $response ), true );
        if ( isset( $data['classification'] ) && $data['classification'] === 'undeliverable' ) {
            $errors->add( 'invalid_email', __( 'This email address does not appear to be valid. Please use a different email.' ) );
        }
    }

    return $errors;
}, 10, 3 );

For the best UX, validate while the user is still filling in the form - not after they click submit. Add this JS to your theme:

// In functions.php - enqueue the script
add_action('wp_enqueue_scripts', function() {
    wp_add_inline_script('jquery-core', "
        jQuery('#user_email, #reg_email').on('blur', function() {
            var email = jQuery(this).val();
            if (!email) return;
            jQuery.post('" . admin_url('admin-ajax.php') . "', {
                action: 'bz_verify_email',
                email: email,
                nonce: '" . wp_create_nonce('bz_verify') . "'
            }, function(res) {
                if (res.classification === 'undeliverable') {
                    jQuery('#bz-email-error').text('This email address is invalid.').show();
                }
            });
        });
    ");
});

// AJAX handler
add_action('wp_ajax_nopriv_bz_verify_email', function() {
    check_ajax_referer('bz_verify', 'nonce');
    $email   = sanitize_email( $_POST['email'] ?? '' );
    $api_key = 'YOUR_BOUNCEZERO_API_KEY';
    $resp    = wp_remote_post('https://api.bouncezero.io/api/v1/verify', [
        'headers' => ['Content-Type'=>'application/json','Authorization'=>'Bearer '.$api_key],
        'body'    => json_encode(['email'=>$email]),
        'timeout' => 5,
    ]);
    wp_send_json(json_decode(wp_remote_retrieve_body($resp), true));
});