I'm not sure if the subdomain screws it up in your case, but my experience with Fastmail is that they handle this really well. I set up a catchall forwarding ad

I'm not sure if the subdomain screws it up in your case, but my experience with Fastmail is that they handle this really well. I set up a catchall forwarding address, and then if I receive an email to whateverburner@mydomain.com, when I reply, Fastmail automatically populates the From address with whateverburner@mydomain.com. Have you set up a catchall address for that subdomain?