Reading package lists…
Building dependency tree…
Reading state information…
Package milter-greylist is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package ‘milter-greylist’ has no installation candidate
Debian 12 dropped the milter-greylist package for some reason. I checked with Debian community a few months ago due to the same issue but got no definitive response why it was dropped from bookworm apart that it was indeed dropped. It doesn’t seem likely it will return to default Bookworm repos, maybe as a backport. Apparently it’s still present in Ubuntu and RHEL distros
As a workaround, you can manually set mail rate limiting in postfix config but iirc it’s server-wide