Send a Custom Welcome Email After Registration

<?php
function custom_welcome_email($user_id) {
    $user = get_userdata($user_id);
    $to = $user->user_email;
    $subject = 'Welcome to Our Site!';
    $message = 'Hello ' . $user->first_name . ', welcome to our site!';
    wp_mail($to, $subject, $message);
}
add_action('user_register', 'custom_welcome_email');