If you’re all set up on Debian, I don’t see the advantage of switching to another flavor of Debian, unless you have a low powered machine (low specs, not much RAM).

dietPi is in fact Debian, with extra scripts to install/remove software. They also thinned it way down, so you get a working system with the bare essentials.

Yeah, snappymail is what I found as the best alternative that fitted my criteria (docker install and unified inbox). Quickly checked out Afterlogic webmail lite, but I am not sure if it does unified inbox?

Interested in learning about the wildcard. How do you set up the DNS to accept that?