The requirement for Debian 12 is a “clean” install, with “no” services besides SSH, like for all other “options” for underlying OS (Rocky, Alma, Centos-Stream).
If you choose a hoster providing a “wrong” config, this must be corrected first!
I am Team Debian, no RedHat relicts!
My 2 cents
Andy
As per the “System Requirements” for installing NS8:
Install NS8 on a clean Linux server distribution, avoiding installation on desktop systems or servers already running other services.
In my book, Exim is a mail service and is even configured as such (sits on Port 25).
Source:
https://docs.nethserver.org/projects/ns8/en/latest/system_requirements.html