I’ve been a happy customer of https://33mail.com/ for years. It’s a different style of offering with a similar purpose and apparently a sustainable business model.
Fastmail supports something like this, but the process of adding a new outbound alias every time I need one is not streamlined enough, so the conversation goes like this:
> otherperson@ABC.com to burner123@subdomain.mydomain.com: Blah blah
> me@mydomain.com to otherperson@ABC.com: Blah back at you!
> otherpersonABC@ABC.com to me@mydomain.com: Who are you and why are you responding to my message to burner123@subdomain.mydomain.com?
Does 33mail make it easy to continue the conversation under the alias?
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?
Under "my email addresses" in settings, I have "*@example.com" as one of the entries. With this arrangement, fastmail properly fills in the appropriate example.com email address when I reply.
It seems easy to miss. I've got multiple domains that I manage this way, and upon inspection at least one of them doesn't have this configured properly.
Yeah... disposable