How can I update the script so that it sees the returned emails, and removes them from the mailing list?

Ask your hosting service if they offer Mailman for managing mailing lists. It takes care of removing dead addresses automatically, which greatly simplifies the management of large mailing lists. Yes, it's written in Python, but so what? It does its job and leaves you free to pursue more interesting things.

Even if your ISP doesn't offer Mailman, they may offer a similar application. Ask them, and find out how they can help you.