Apple are pretty clear this is for one way communication. It’s not an email replacement, it’s a privacy tool. There is always a trade off when it comes to privacy, and in this case it is that you can’t send email.

There are plenty of great tools out there which allow you to create multiple addresses with the ability to send. Maybe you should consider the trade offs that are acceptable to your specific use case and invest/use/setup/buy into them? You could buy a random domain and set up a catch all address if you want to keep it simple.

Before Relay was a thing I wanted to learn about SMTP and make a service which could do something very similar to what Relay ended up being but for mobile. I’m still running it ~4 years later (https://inboxesapp.com) and it’s a great learning experience. If you don’t have what you want - build it!