I can confirm that courier-webadmin does not work. On a fresh Debian 12.4.0 install (32-bit, server setup), after immediately installing courier-webadmin, when trying to access the webadmin frontend thru http://localhost/cgi-bin/courierwebadmin as stated in the install wizard, I get a 404. Plain "localhost" loads the Apache default page as expected.
The upstream website is also down quite a lot, and has to be accessed through the Wayback Machine. It's hard to get manual configuration instructions, and if I'm not sure that other components are functional, I doubt I have the skills to get it all working. A partial substitute, with a mail server, webmail and webadmin, would be the Freedombox package: https://tracker.debian.org/pkg/freedombox Unfortunately it's not a very exact substitute. Freedombox doesn't really seem to have multi-domain hosting support; it has to be kludged to host a second website on a second domain, and I'm not sure if one could kludge it to do what Courier calls "vanity fronting", where each domain seems to have its own mailserver, webmail, users, etc.. Nor can Freedombox be set to provide just a mail system and allow other programs to provide other server services; it's intended as a monolithic standard-form home-server app that is administered entirely through a web interface, which is useful, but not at all the same thing. Is there any other mailserver package that plays well with Debian and is easy test and maintain?