You can do this without masked email too. I have a wildcard alias setup so *@mydomain.tld are accepted, and I have a wildcard sending identity *@mydomain.tld
If you have a sending identity that matches with the address the email was received, on the app and webui it will reply from that address.
Disclaimer: I work at Fastmail, but not on the webui or app.
Fastmail does SUCH a great job managing multiple addresses and domains. My favorite feature is the auto-BCC so that when I'm sending from a joint e-mail address I share with my wife, she always gets a copy.
I've been trying to decide if I should switch from GApps to Fastmail. This is the killer feature I wanted along with unlimited domains, looks like I'll be transferring over soon.
If you have a sending identity that matches with the address the email was received, on the app and webui it will reply from that address.
Disclaimer: I work at Fastmail, but not on the webui or app.