I currently do exactly what you describe, for the purpose that you describe, with fastmail. This feature is called aliases on their system. I also used to do b

I currently do exactly what you describe, for the purpose that you describe, with fastmail. This feature is called aliases on their system. I also used to do be able to do it with migadu when I was hosting with them. The aliases are routed with a longest-prefix-match sort of algorithm, so you can blacklist specific addresses that have gotten spammy by aliasing them to some inexistent address.