Get woocommerce coupon code

You could use the woocommerce_applied_coupon hook. You find the “implementation” in the class WC_Cart (
method apply_coupon)

Usage example:

function my_little_coupon_func($coupon_code)
{
    echo "coupon is correct: ". $coupon_code;

    // now send your email

}

add_action('woocommerce_applied_coupon', 'my_little_coupon_func');