Do you use an external DNS when accessing your subdomain? I can only guess that it's the DNS leaking it.

Sounds great! Is there an easy to follow/understand tutorial on setting up HAProxy node?

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.

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

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).

The problem with such a situation is that you have no idea of the origin IP address, as all the requests look like they are coming from your VPS. Did you find a way to restore origin IP in your logs?

Sorry for the late answer, but will your SMTP logs show the original IP or the VPS'?

How do you do that? (Reset on kernel panic)

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?

Ok for http trafic, but what about SMTP?